I thought about using screen sharing from a support perspective when I posted it in the news this morning, and it'll seems kind of niche. It'll definitely come in handy if you're helping someone with a specific app that you're not familiar with, but otherwise the consistent, predicable nature of iOS means that they won't have, say, a window in a weird spot on their screen, or not see something that should be there (all other things being equal).
I.e. you'll always be able to tell someone to tap on the time, for it to scroll to the top of whatever list they're looking at. They'll always be able to press the home button to go back to the home screen, etc.