I'll add my two cents in here.
I believe that it depends on what you are studying the martial arts for as to when you should cross train (or whether you cross train at all.)
If you are studying for the martial aspects... i.e. you are anticipating that you will be faced with combat soon, you should probably cross train as soon as possible to cover the situations you might be faced with. (Kenpo for stand-up, BJJ for ground, Arnis for weapons and etc.)
When studying for the martial aspects, efficiency is key. You want to learn as quickly and as effectively as possible because you feel that you face a threat that could come at any moment.
If you are studying for the artistic aspects... i.e. This is a hobby, lifestyle or a form of recreation, it is my opinion that you should probably wait until you at least have your 3rd Degree Black Belt.
When studying for the artistic aspects, the goal is not only to know how to use the material but how to incorporate it into who you are as a person. This means that you should know enough to not only understand how to use it but you can analyze it and break it down to the point where you can understand why it works for you and why it might not work for others.
Because we all focus on different things we have different opinions about whether cross training is important or whether or not one should practice forms. The martialist is focused on defending him or herself and that is the sole focus. All other gains are secondary. The artist is interested in the big picture, they want it all and are willing to take their time to get it.
Both approaches to Kenpo have their merit. Some truly need to have the martialist approach, but once they have their brown belt in Kenpo and a purple belt in BJJ they should have sufficient skills to protect them from an attack. Once they've won or lost that impending battle, where do they go from there? Unless they decide to move onto something else, they must move from being the martialist to being the artist. Most don't make that transition very well. The whole artistic approach just isn't them. They've focused on efficiency and effectiveness that the artistic approach, seems wasteful. Forms? There are drills that teach the same things as forms but more efficiently and more effectively. So? To the artist a form is done not only because it teaches something, it provides a sense of enjoyment. And so the martialist moves from the primary art that he/she loves and moves on to another one because it is effective and efficient. In time they will begin to see redundancies and inefficiencies in that art and will move on to another one that is effective and efficient in a different way. The martialist will spend a lifetime training hard to be unbeatable because there is always the realization that there is someone out there better and want to be prepared in case an encounter occurs.
The Artist approaches the arts very differently. They have experienced one art and even though there may be other more effective and efficient arts out there, this is their ART and they love it because it provides them with everything that they need. They are interested in learning self-defense but they have a need that good beyond just self-defense. They love the motions involved in their art and they continue with it because they know that if they put in enough time they will be able to understand so much because all of the martial arts are based on one common thing; motion. If they continue to analyze and study motion and the principles inherent to motion in time they will be able to understand it all. If they don't then the journey wasn't wasted because it did lead them somewhere and that is all they want. They spend a lifetime constantly being amazed at all of the tiny aspects of the arts that they have to learn. They can't comprehend the martialists approach... why would someone go somewhere else when you've got everything you need right here? The issue is efficiency. Yes, the artistic approach of self-discovery winds up being more rewarding artistically than the martialist approach of seeking out others who have already made the discoveries, but the martialist approach makes for better fighters sooner.
Is one way better than the other? It depends on your goals.
When should you cross train? It depends on your goals.
That's my opinion. It's also my opinion that it is silly to argue over such things. It all has merit, depending on your perspective/point of view. Go ahead and debate the issues and try to help the other to understand your point of view, but understand that it may not be possible and that no matter what you shouldn't take the issue personally.
Doug Turner
A side note: No matter who you are, if you've learned something from more than one source you have cross trained. Cross training is merely the study of motion from different perspectives.
