Well a couple of thoughts. When we talk about cross training in sports, each sport has different skills and different purposes (except winning of course
). In martial arts we are dealing with how do you respond to an attacker, and this is mostly regardless of the martial art you do, so while I understand the cross training comments for sports I'm not totally sure it fits with martial arts.
My main thought though is that while there can be real value in martial arts cross training at any stage in the training, when you are really grounded in an art you start to be able to intergrade other arts in to that art more naturally. I'm sure there are folks out there who are able to do that early in their training, but I certainley was not Kinetically gifted enough to do that until I was much further into my Aikido training.