Those who think in general-election terms might see this as folly. But Edwards is running in the Democratic primary and this is a great opening for him. Of the cable news networks, MSNBC probably garners more active liberal voters, so it's a smart place to buy an ad that won't be seen by center-right voters and general-election people he'll have to worry about switching (or at least not offending) later on down the line.
It's also a deft blow, hitting Hillary Clinton and Barack Obama where it hurts. Both of them can frown and say they disagree with Bush and Petraeus, but it remains to be seen whether they'll have the, uh, stones to cast a vote that might come back later to bite them.