Romney's endorsement is certainly beneficial to McCain.
Romney won caucuses, primaries, and millions of votes in the campaign.
McCain should secure the nomination outright sometime in March, leaving him free to unite the party, and to campaign full time for the Presidency, while Obama and Clinton are still battling each other tooth and nail to the convention, in a potentially bitterly divisive campaign.
McCain should go with Romney for VP.
Romney would terrible VP: he's stiff, overly slick, and not likeable; living proof that you cannot buy an election.
And anyway I highly doubt Clinton vs Obama will go to the convention: Clinton will run out of money at some point, and my guess is that she'll drop out or suspend her campaign sometime in April or May after she loses court challenges to seat the FL and MI delegations and the superdelegates defect en masse. Even if she doesn't, it will be clear to everyone that she won't be the nominee, so Obama will be free to swing into general election mode.
caveat: all this is dependent on Obama winning the WI primary and HI caucus. If he loses one or both, Hillary becomes the heavy favorite to win the nomination.