Because in three way races, sometimes front runners take each other out by training their guns too hard on each other? It's sort of a version of what a lot of Democrats were worried about with the extended primary in 2008 possibly allowing McCain to slip in. Incredibly wrongly worrying about it, as it turned out, but it's been known to happen.
I think no matter what the President's intentions are, the left should be pushing their agenda the same way. If Obama's goals are perfectly aligned with the progressive movement we lose nothing, and if they're not we move them in a constructive way.
Besides, making it known that there's a large and active part of the population that wants things like GLBT equality and real healthcare reform helps the people in power who are already with us.
I think there's way too much talk about whether or not the President is 'on our side'. The people primarily interested in that are mostly seeing politics as a soap opera or want to establish their moral superiority, instead of actually doing constructive things.
Your memory can't possibly be that selective. He was ridiculed in the REPUBLICAN primary - just to show how far back it goes - by Mitt Romney for "being Jane Fonda[Iraq] one minute and Doctor Strangelove[Afghanistan and Pakistan] the next". The whole thing about cross-border attacks at hot targets in Pakistan was called naive by both John McCain and Hillary Clinton. I'm sure if I checked the blog history I can even find a diary by you agreeing with her, since you always did.