Sue C over at My journey to black belt asked this question in a post found here. I recommend you read the post and the comments as I'll be referencing some of the material.
I've gone back and forth on issues related to this question for quite some time.
First off, I have cross trained in other arts. I have done this both out of necessity and by choice. My experiences, for the most part, have been positive, but not all of them have been.
Sue talks about some of the potential advantages and disadvantages of cross training for kyu belts. She cross trains and has had positive outcomes. She is also grading for black belt this year and has been a serious student of Karate for several years. See her blog for more information.
Sue lists the following as advantages to cross training:
1. Core principles. Sue notes that after dedicated study, you recognize similarities between different arts and their principles.
I cannot agree more. The longer I study and the more people I meet in the martial arts, the more I realize the similarities. As Sue mentions, it is most often a matter of what area of the art is emphasized more. Core principles of movement, reaction, body mechanics etc. have tons of overlap. If you train with a mind that can identify similar principles, then you can open your mind to different perspectives on those very same things. A different point of view can change your opinion, alter your technique or strengthen your belief in what you've already learned.
2. Talks about attention to detail. Sue take Karate as her main discipline but supplements it with Kobudo and some Jiu Jitsu. The arts all contain many of the same techniques, for example, strikes, but each spends differing amounts of time on this type of technique. In her training, little time is spent on the mechanical breakdown of strikes in her Jiu Jitsu, but much more time is spent on it in Karate.
Different prioritization on techniques is definitely out there. Where as I might argue that repetitive striking is less important than something else, a dedicated Karate practitioner could counter that one proper strike could easily end a confrontation. Sue mentions an advantage of her cross training is that she has learned to throw, be thrown and break fall with far more confidence than she had before. Tough to argue that.
3. Sue expands on differing perspectives. She talks about 'hard' vs. 'soft' styles and what that means in application.
As many discover, 'hard' vs. 'soft' is a bit of a misnomer. The assigning of a hard style, such as Karate, or a soft style such as Jiu Jitsu can be misleading. The techniques can be very similar. To over simplify it, it refers less to the application of the techniques, but more to the approach or entry to it. Karate is more of a 'force meeting force' type of style where as Jiu Jitsu is more of a 'yield to the initial force and redirect' style of martial art. While this holds true, anyone who's spent time in either art realizes that there's lots of overlap.
4. Enhanced understanding of certain principles. Sue discovered that some of her core techniques from her main art improved while studying weapons and weapons defenses.
This is also important. If we're not careful, we can often go through the motions on certain techniques, especially if we've done them for a long time. Sometimes shaking things up cause us to re-examine what and how we're doing things to ensure what we are doing is effective and correct.
Sue touches upon disadvantages to cross training as well. The main issue raised is the differences between technique, whether it be stances, break falls, movement etc. These can conflict if your styles have different perspectives on things.
The subject of cross training is a tricky one. In Sue's articles, there are quite a few comments on whether or not kyu belts should cross train in different arts. They range nearly from fully supporting the idea from day one to feeling it shouldn't occur until at least black belt. Actually, each camp had good points.
My feelings are mixed. As with so many topics, it depends on the student, the teacher and the martial art itself.
In my opinion, one of the most important points Sue made was that she had a 'core' or a 'main' art. For her, it's Karate. To her, she gains benefit from cross training mainly because it improves her Karate. That's not to say she isn't or will not excel in the other arts, it just means her focus is mainly on her chosen art.
This is very important to gain benefits from cross training. If your main focus is not on one or the other, both tend to suffer.
If you are undecided, it will forever be a battle of which style is superior, correct or more sound in theory. This conflict is not a good idea at lower levels of study. Flitting from one style to another doesn't help either. If you leave one art before you really understand it's core concepts, you do yourself and the art a disservice.
Find an art that works for you, makes sense and has a good teacher and students. Once you've done and committed to that, see what else can help you.
I think your motivation for cross training is a key point in this debate.
I'm lucky enough to have a Sensei who encourages cross training. He's always interested to hear what else is being taught, how and by who. Of course, he is comfortable in the knowledge that my main study is with him. Having said that, he has also encouraged all his students to cross train and has supported anyone who chose to leave for another style. Such is his comfort level with himself and his style.
In my style of Jiu Jitsu, my Sensei and the founder of the style have always supported cross training and always have an open mind to new ideas or techniques. I've taken seminars and it's always great fun to come back with my new knowledge to explore it further with my Sensei. Sometimes he incorporates the material if it works well, and sometimes he doesn't, but it's always after an open minded investigation of it's effectiveness. Some teachers are threatened by new or differing ideas.
I'm also lucky that my Sensei never wants there to be carbon copies of him as black belts and beyond. In fact, after a few years of serious study, students are encouraged, or even required, to make the style and techniques work for them. Every person is different and must find what works for their body style, age, weight, height, strength, speed etc. This open minded approach encourages learning outside the dojo.
The trouble in cross training lies mainly with the inexperienced. For a time, stances, footwork, strikes, blocks, break falls etc. are either done correctly or incorrectly. If I adjust my style now to suit my personal style and abilities, that's a good thing. For a new artist, varying from what is shown or taught is incorrect. Like I've mentioned in previous posts, you must learn and understand something before you can determine if it should be discarded from your repertoire.
Some say you should train in a complimentary style. Not a bad idea but it can be limiting too. After all, if it's more of the same, what benefit are you gaining? (other than additional training time). The flip-side is that if the styles differ too much, you just get confused.
I am very interested in knife defense. I have supplemented my Jiu Jitsu training with Kali and Escrima to improve my skills and understanding in this area. I've enjoyed and benefited immensely from this training. At one point, I tried to move beyond supplementing my Jiu Jitsu to becoming a full time student of both arts. I ended up struggling with this balance. Specifically due to distance to my opponent, stances, offensive versus defensive etc. At that point my Jiu Jitsu was so ingrained that some aspects of my movement and reactions could not easily be changed. Nor was it advantageous to do so, for me. I ended up fighting my training so much that I was spending more time trying not to do what I'd trained myself to do for years that I wasn't benefiting from the new knowledge. And it was affecting my Jiu Jitsu.
As a result of this, I stopped training as a new student. I still train with Guro to improve my knife technique but now he and I tailor it to my needs and my body and training. He's so talented that this is easy and he and I both understood that to try to unlearn what already works well for me was counter productive. I mention this as I still needed and benefit from cross training with Guro, but now it's to make me better in my Jiu Jitsu. For this reason, I think you need to have some time studying something so you can have the confidence and understanding of what can benefit you on your journey.
I've also cross trained out of sheer necessity. For a couple of years, I was on shift work that prevented conventional 2 or 3 nights a week study. I could only make 3 classes a month in my chosen art due to my schedule. So I went to another club that had classes on different days and at different times. I was lucky to find another Sensei that understood my issues and welcomed me to his club. He appreciated my background and didn't feel threatened by differing opinions (I was lucky).
To sum up, I believe cross training is beneficial for most people. I do agree that some dedicated study is beneficial before branching out. I believe that when you find an art that works for you and that you enjoy, search to improve yourself in that art.
I will continue to cross train to improve myself, but at the end of the day, I'm a Japanese Jiu Jitsu man through and through.
I'd love to hear any opinions on this topic.
Train safely and with an open mind.