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.