Well Rob, I enjoy a good debate as well, so here you go!
While I certainly see where your line of thinking is coming from, I think the major problem with an entertainer being chosen over someone like Mother Theresa is motivation. While it's true that the examples you chose did make peoples lives better, I think for the majority, their major (in some cases, only) motivation for their actions would have been money.
Sure, entertainers do great things, but it's not exactly hard to do it when you're getting a six-figure paycheck, is it?
Mother Theresa lived in poverty (just about) but still helped people her whole life. That's what makes her more deserving of the award than someone like Jerry Seinfeld, or Steve Martin.
She was willing to give everything to help people. While entertainers may be fine people, you have to wonder wether they would still be doing it if it wasn't profitable anymore...
