I think it's very important for most of your career but becomes less important after a certain level of success has been reached.

Once your level of greatness has been determined than it becomes more clear how long you need to stick with one trainer or if you are good enough to bounce around and still maintain your skill level.

I think of weight training. The idea is to have a trainer teach you how to train so that eventually you can do it on your own. A lot of people never gain the confidence they need to proceed without a trainer overseeing the entire process. I see training for fights the same way, except for sparring and hitting the mitts, you need another body for that.