Many of my DVD purchases have been made after viewing samples on YouTube. Similarly, if I'm booking tickets for a company I'm unfamiliar with, I'll look up clips to find out which dancers I should try to see. I recently spent about twice as much as I planned on tickets for the Bolshoi visit to London as a direct result of watching some amazing clips on lexxrussia's channel.
I can't suggest any legal or moral justification for allowing copyright stuff on YT; just the pragmatic one that it may not be hurting the copyright holders as much as they think.