Presidential primary politics is not, as Robert Wright might put it, always a zero sum game. Given the prevailing metaphors of political positioning and space, one candidate often fills an ideological space and displaces others -- think Pat Robertson in the 1988 Iowa caucuses. But just as often, the power of one candidate in a particular state provides an excuse for another candidate to play down the stakes.
To put it one way: Mitt Romney needs to have a strong Christian conservative candidate in Iowa. Ex-AR Gov. Huckabee's grassroots supporters have been working Iowa hard, and Romney might have gotten a pass if he decided not to campaign in a state where his religion was a barrier to a victory. A "pass" doesn't mean that Romney wouldn't have to compete in Iowa. He would. But he could use his existing, fairly formidable infrastructure to take home a solid bronze, fulfilling expectations.
Without Huckabee -- if he decides not to run -- and assuming Sarah Palin does not run or does not gell with Iowans -- Romney's ability to half-heartedly compete in the state and defer to the clear favorite is minimized. Romney would face expectations that he at least match his performance from 2008. If he doesn't think he can do that, he can try to opt out entirely, which creates its own set of problems.
Just a reminder that Huckabee's pardon woes have more than one dimension to them.