The only thing Brady has left to prove is that he can do it without Bill as his coach... but really that seems like a minor thing to me. No matter what Brady will have to be looked at as the greatest QB of all time, or at least one of those names that has to come up in that conversation.
Rivers is the QB of his class without that SB ring, and if he wants to see himself in Canton someday he will need one of those, he is coming off of a year where he played well so there is no reason he should not expect to be able to perform for another team somewhere. But he needs to land with a contending team, no way he signs somewhere that is not ready to win this year.
Roethlisberger i could see wanting to have a send off game like Eli got, plus I think a couple of his stats got passed up this year and he might want to scratch them back. I am not sure I could imagine him in any uniform other than the Steelers though, not with his injury and piece of shit history.
These QB's (and Eli really) also have that possibility of keeping in shape for around week 8 when some top team loses a QB and they are desperate for a Veteran to step in and win some games.