I'm a performing arts teacher so obviously I completely disagree with your stance!
I think the plays we put on at school are so valuable for the children. Our current production has 6 large female parts, 5 large male parts and includes 72 children altogether, all of whom are in at least 2 scenes. There is no prima donna and, of those 11 principal parts, 3 are being played by children on the SEN register, 2 have aspergers/ADHD and 5 of the children are unlikely ever to gain any recognition for anything else - mainly lower sets, not first team sports etc but awesome singers and surprisingly mature actors. Okay, the other 6 are children who seem to do well in everything but I think we've managed a good balance!
The school where I work only takes children up to 13 but (in as unbiased a way as possible
) we put on bloody good shows! Certainly better than anything I was in at secondary school. It isn't because we're awesome teachers and it certainly isn't because the children aren't busy with other things, it's because they are genuinely passionate about it and love what they are doing.
Yes, some other teachers get snotty about rehearsal commitments and children who need extensions for homework etc - but in terms of the children's development and enjyment I think the plays are well worth it.
It's not like other (or the same!) children don't have constant demands on them from sports staff!