I think the biggest problem is this: These kids come to high school with a reputation of having a certain level of talent. When they are passed over for roles, for whatever reason, the other kids at the school begin to falsely believe that they weren't so talented afterall. Especially when kids with very little talent beat out the professional kids. My daughter says that it is just embarassing. Only her close group of friends has seen her perform in outside theatre.
The saddest part is that teachers are supposed to inspire kids to follow their dreams. Their job is to seek out those with special talents (whether it is math or science or art) and guide them, mentor them, and support them. There is nothing worse than blowing out the candle in someone's heart.
But, here's the good part: Everyone needs something in life to drive them. And, if my daughter wants this bad enough, she will use this as fuel to succeed.