If you think about most of the debate structures, they single out the "front-runners" with the more controversial topics, give them first shot at answering the questions, while the others have time to reflect, collect, and prepare for a response that may be unique to the others or bring out additional things to think about.
I think this is a big reason why Gingrich shined in the early debates when he was a second-tier candidate, and it helped him to appear more likeable and less "mean" to the other candidates.
Romney has pretty much been a top-tier candidate since the beginning, and has thus been involved in ALL of the spats and difficult questions as a first responder. Overall, that hasn't helped him. He's a good debater and has handled the questions effectively for the most part, but he's still been exposed to the scrutiny he wouldn't have if he was lower in the pecking order.
I also think this is one reason Bachmann had a good debate the other night.