Re: Cross Discipline Training
I think most of you are missing one point: Phases of development.
Personally, I do believe cross training to be a positive thing, but not at every stage of learning.
During my fourth year of studying Aikido, I joined my teachers Karate group too (my teacher has Korindo Aikido, Karate and Judo groups and has studied all of these 3 arts in depth - above 6th Dan level). After some time, some of my Sempai told me the Karate studies are hindering my development in Aikido. I lost the softness I was developing and became stiff (a better contrast to "soft" in the M.A. sense). This was one of the reasons I decided to stop the Karate.
A few years down the line, I had free time once again, and added TKD with another teacher to my studies, in addition to Aikido. This time I only found my horizons widen, and the TKD increased my Aikido (my Aikido habits were very problematic for some of the TKD practice such as Pumse/Kata).
The same hold true for any other type of cross-training. It is not generally a wrong thing. But at some points in your development, it may be more of an obstacle then progress. A good teacher may therefore ask you not to cross train during those times.
Another bad example could be a beginner who starts learning a technique and then tries to utilize it in a cross training sparring, fails and thus changes the way he does this technique - so it will work better. In theory - this is a great way for development, in practice, the beginner has yet to really learn the technique and the opportunities for it, before he starts growing out of it.
Thus - cross training is great, but should be done correctly and appropriately, taking into consideration you development.