Yes, the arguments against this rush to mind:
1. Romney is a member of the opposition party... but that's also a plus: We could have a real show of bipartisanship and of genuinely wanting to make this legislation work for the benefit of the American people, not just to make it look good at the right times and in the right ways to bolster the power of the Democratic Party.
2. When he was running for President, Romney said he wanted to repeal Obamacare... but he was working with the preferences of the GOP base and needed to win the nomination. He distanced himself from his history of making Romneycare work by saying he'd been Governor of Massachusetts, working with the preferences of the people of Massachusetts. This suggests that he's a practical, able man who understands the task he's been assigned and sincerely applies his expertise.
3. Romney wouldn't want to help Obama, who defeated him in the election... but Romney's image is of a public servant who wants to do good works for the benefit of all. He isn't — or at least shouldn't — want to seem like a guy with a grudge.
4. Romney should stand clear of the spectacular collapse of Obamacare. Really? Isn't this like his signature achievement, saving the Olympics? The magnitude of the disaster and the importance of avoiding it are exactly what should attract Romney. "He saved the Olympics and he saved Obamacare" — wouldn't he want that legacy?
5. Obama wouldn't want Romney showing up on the scene now as the savior. Obama is too narcissistic, too peevish to call out to the older man for help. Maybe. But Obama would be the President, and he could look very magnanimous and wise. He could finally seem to be doing what a lot of us thought he was about when we voted for him in 2008 — bringing us together.
6. Everyone, right and left, would cry out in horror or at least puzzlement. And what a distraction that would be.