The idea that the 2012 primaries harmed Romney's prospects in the general election deserves more scrutiny.
What scrutiny?
Rather than letting Romney define himself and what the Republican Party stood for going into 2012. It put on primetime television from 2011 to mid-2012 a circus of far right jokes to define the Republicans for the American people. Obama could just tie Romney to being the leader of the party that would put Rick Perry, Herman Cain, Newt Gingrich, Michelle Bachmann, etc. in power.
It also created an incredibly conservative delegate turnout at the GOP National Convention which led to Romney's acceptance speech looking a Nazi rally rather than something to be proud of.
It's worth distinguishing between effects from business plan candidates like Gingrich and effects from competition itself. Party leaders clearly believe that the former wasn't helpful to the party at large - hence their concern about excessive debating - but my skepticism holds in either case.
More to the point, is there any reason to expect that "letting Romney define himself and the Republican Party" would have been anything other than a disaster? It would have been a case of giving a condemned man enough rope to hang himself.
