@JPB,
The primary season does offer actual benefits too though. How many people have seen Romney or Gingrich in person, shaken their hands, looked 'em in the eye? That's genuinely powerful, and some other person swooping down from on high might just not have the same capability of building up that sort of connection.
Not to mention just plain on-the-ground operations, county offices, volunteers, all the rest of it.
The convention is August 27th -- these days, ten weeks is just ridiculously short for a viable presidential campaign.
Again, I think that a core (35-40%) of voters will just vote against Obama no matter who they're voting for, and this stuff doesn't matter for them. But to actually win, it really matters.