Main Idea (own words): The reason Romney didn't get much of a bump, if one at all, because he had long ago earned the majority of the republican votes.
I agree, the fact that Romney got a very small bump in polls is because he had already consolidated the majority of the republican vote. So even if he had consolidated the majority of the votes he had not already, it would make a slight difference. Not one that impacts his campaign in any major way. So although I'm sure it helped some of those who were still wavering with their vote decide which way they wanted to go, the number of such republicans was a small minority beforehand.