Lindy,
It is quite possible that DD will go to dance college at 18 (not before - she needs the fall back of the very high grade A-levels she is capable of getting, and the local dance school is entirely serious enough to get her to the standard required for the 'general' dance colleges). Or she may not, but the work ethic, the belief that it is worth hours of practice to perfect a 5 minute performance, the time management skills to keep her school marks high and her school effort marks at 1*, the teamwork and friendship, not to mention the exercise, are things that will stand her in good stead whatever she does.
It's not a question of 'putting up with it' - that amount of work is needed for her to attain and maintain the standard that she has reached, and the competitions 3x a year are where she does the ''performance' part of what is, after all, a performing art.
If she wasn't enjoying it, or wasn't improving, that would be different. But if you go over to e.g. the musicians thread, they too put many hours each week into lessons, practice, orchestras, groups, concerts, and nobody says 'well, i wouldn't put up with the amount of commitment needed to attain a grade 8 distinction in music'. Equally, doing well at school takes many hours of work, or achieving highly at a sport required a daily input of a certain number of hours. Why should I limit DD's attainment in dance in a way i wouldn't limit DS's attainment in music, or either of their attainment in academic subjects, just because the dance world is an unknown one to me and the music world a known one?