A simplified early history of martial arts in the US and in Canada (1940s, 1950s and 1960s) is that one could only train in one martial art style or otherwise be seen as deviating from the truth or be seen as betraying your sensei/
The idea of cross training has constantly been on my mind for many years. One has to be careful when cross training. One would have to keep in mind the cliche “jack of all trades and master of none.” I think that to make cross training work, one would have to do so in an intelligent manner. One cannot just study several martial art systems, cherry pick a technique here and there and give it a new name. I think that you must have an overall strategic idea of what you want to do. First, I think that it is imperative that a student have a base art. I would go so far as to say that the student should have at least a black belt, or a substantial experiential equivalent, in the base art before embarking on a cross training journey. That presumes that the student and the base art fit each other well. For example, a tall lean flexible person might be better off studying TKD and achieving black belt before cross training.
Setting that issue aside, once a student has sufficient experience in the base art, he or she must decide how to approach cross training. It is my belief that every martial art system has a weakness. Don’t buy any of that “ultimate” system crap. There isn’t one. To use the above example, the TKD person might decide to pursue different avenues depending on his or her preferences and needs. The TKD person could pursue a system that emphasizes ground fighting, or one that emphasizes joint locks (aikido or small circle jiu jitsu, for example) or one that emphasizes in close fighting (such as wing chun or JKD) or a weapons based system (such as iaido, kendo, or filipino martial arts) There are many choices.
Here is the tricky part. I really do believe that once a student starts cross training, one should stick with the system he has chosen (after an investigative phase to determine the next course of study, of course) and take the time to integrate it into the base art. In my case, I started out with Cooper Ryu Vee Jitsu and have concentrated on Modern Arnis for the last decade. These two blend very well. There are other aspects of the martial arts that I would love to study….such as groundwork and pressure points. However, I am not in a rush.
The best martial artists that I have seen out there are those who have cross trained. Why? Because they are more well rounded and are better prepared for any contingencies. Just look at the MMA scene. The best fighters are those who are equally adept at kicking, punching, kneeing and the ground game. If you are weak in any one of those areas, your weakness will be brutally exploited.
I could write more on this topic but I will stop for now. To summarize, cross training, done properly and intelligently, can be extremely beneficial, not to mention expanding your martial horizons. Yet, at the same time, you do not want to become a jack of all trades and master of none.