First and most importantly, it would force the various political Hamlets out there immediately to quit or to enter the race.
Second, like him or no, RG would be one of the few adult candidates, and would make the GOP debates not only more interesting but of greater value to the primary voter. I would expect him to make mincemeat of some of the weaker ones, and prompt their exit before they do any more damage to their own future, or the party's prospects in 2012.
Third, having to deal with a strong, well-known contender like RG would demonstrate how capable a candidate Mitt Romney, the current GOP front-runner really is.
So far, MR has failed to impress. Larry Sabato and Kyle Kondik opine in their piece, GOP 2012 Update: The Big Tease
Leading the first tier of contenders is former Gov. Mitt Romney (MA), who is an exceptionally weak frontrunner, perhaps the weakest GOP frontrunner since 1964, when the GOP rejected Nelson Rockefeller and other moderates in favor of Barry Goldwater. We all know how that one turned out. Romney has many problems: He is seen as a flip-flopper — a one-time moderate turned conservative warrior. Even worse, his signature accomplishment as Massachusetts governor was a health care plan that provided the model for President Obama’s plan, which is loathed by most Republican primary voters. That said, remember how John McCain supposedly was so disliked by the GOP base that he could never win the nomination? We all know how that one turned out, as well.For contrasting local views on MR, check out two Boston Globe op-ed articles by Republican consultants, Dan Payne's Romney Can Win GOP Nomination and Todd Domke's Can Romney win ? No: Voters want the real deal, not pandering and positioning.
For a potpourri of opinion, not necessarily impartial or informed, on Giulani in 2012, check out these comments in THE POLITICO's Arena.