Toesucker has it right:

But at this point, the practical impact of any support for Edwards is to help Clinton defeat Obama. Polling shows that the second choice of Edwards' followers is overwhelmingly Obama.

By staying in the race after he has lost any realistic chance of winning, Edwards is making it possible for Hillary to win and to bring with her precisely the policies he most opposes: dependence on special interests, a determination to maintain a presence in Iraq and a reversion to dynastic politics.

Any more thirds and it's time to get out, John.