I think you're absolutely right that Obama has the potential to be one of the few candidates who can so entirely transcend the netroots--and even the grassroots--that he has a chance of being elected via national media alone. That's one reason his candidacy frightens me. (Though, of course, I'd fight hard for him in the general.)

I much prefer a candidate who is beholden to the grassroots.

And the very problem is that a lot of people think that leftie strawmen are true. That's why he shouldn't be perpetuating them. If his strawmen were things like, 'Some on the left say we must balance every budget even if that means slashing all social programs to the bone,' then they wouldn't be as objectionable.