Think of it this way, if Britain leaves that only leaves two significant net contributers to the EU project (Germany and France), how long before one of them decides it doesn't want to pay for the rest of Europe? What happens when the Euro (as a currency) fails? 28 nation states all looking for cheap ways to transfer value around.

It's a bit like three rich guys going to their favourite resteraunt, and inviting all their not so rich neighbours to come along. Britain is slipping out before the bill arrives :)