It would greatly help out the sport to penalize excessive holding (the heavyweight division would benefit the most) but holds that break themselves up are completly legal (to my understanding anyway). The only time it becomes an issue is when the ref has to break it up constantly because this gives the fight a very unprofessional and boring feel. There should be stricter rules for it but its only really a problem in a lot of the heavyweight fights and a few select people in the lower weights.