Do they sway voters? Probably not. But they definitely sway media narratives.
In light of the endorsement from Chuck Hagel's wife today, the question has been raised about Chuck Hagel's rumored endorsement (and Colin Powell's). Clearly Nebraska's 2nd Congressional district electoral vote is in play--witness Sarah Palin playing defense (and then lying about it) on Sunday.
But the one constant in the Obama camp is that they have been masters of timing their endorsements perfectly: Ted Kennedy, Bill Richardson, John Edwards.
Chuck Hagel endorsing Obama will be a big deal--media-wise. My guess is that the Obama camp is saving that card for if and when they need it. (It would be pretty effective for Colin Powell and Chuck Hagel to endorse Obama after a Bin Laden videotape emerged, for instance.)