My thoughts are similar to Alex's. Maybe when people say support, what they mean is product development support. Customer needs are continually changing and the ILS software has to change to meet those demands. If you don't have a programmer on your library IT staff, and your software vendor isn't doing product development, you are left dependant on other libraries' IT staff to develop your ILS. I think that makes people uncertain about the sustainable future of an open source ILS.
Using the owning vs. renting analogy... when the hot water heater goes out, you just pick up the phone and your landlord sends some one out to fix it. When it's time for a new roof, that expense comes out of my landlord's pocket. Renting is great when you've got a great landord. Most ILS vendors are really bad landlords, but not all of them.