My daughter is a competative swimmer and they do Yoga two days a week. I have no problem with it; I see Yoga on the same level as Cross Fit and other programs that increase strength and flexibility.
Athletes need to be in tune with their bodies and Yoga provides that.
Now if the purpose is spiritual, such as you would find in a church, then I have a problem with it.....
ballet is better for that stuff, core and balance and you don’t get the screwing with your mind that comes with yoga