I feel like limiting the number of teams a child can perfom on doesn't hurt anyone (gym, athlete ect). Percentage can hurt some of the smaller gyms that need to be able to have the option of crossing kids to be competitive. Don't get me wrong, I absolutely HATE crossovers. They are more trouble than they're worth. However, I've also been at gyms where they are a neccessary evil. For me, I much rather limit the athlete. It keeps a kid from competing on 3 (or more teams). Keeps them from getting burnt out, saves them from possible injury, and potentially increases their performance value to the teams.
This is all just my personal opinion. I completely respect those who will disagree with me. Like I said, I don't like crossovers, but I understand that some gyms need them to survive.