It happens all the time, I really don't see what the big deal is. And yes, my boss would allow me to take off time to do my job for somewhere else -- it looks good for her that her employees are in demand. (I wouldn't even lose my pay.)
He won't be collecting a paycheck from Cinderella that week (I'm assuming he'll need time for rehearsal, as well) -- unless he uses vacation time.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.