While I don't necessarily agree with the notion that you don't get anything in return when you give away money, let's assume that it's true for the sake of the discussion.
It tends to be difficult to get people to spend money even when they do get something in return. Many charities and projects run on donations only, and while many of them fail, so do commercial projects where you pay to get "something in return". I don't think the problem has anything to do with pledge systems.
