I mentioned earlier I have an article in the new Washington Monthly about one of the key things worth knowing about presidential elections: candidates actually do carry out their promises. I go through the political science research, and tell a few stories -- such as how the context of the 2000 nomination battle pushed George W. Bush to unveil a major tax-cut initiative, which of course he then went on to fight for from the White House.
That's why the remaining Republican primaries and caucuses matter, as long as the nomination is still being contested, even if Mitt Romney is overwhelmingly likely to win. As long as he still has to fight for it, he's going to be pushed to take positions that will appeal to the various primary electorates, state by state. And the structure and context of the thing can matter. A Romney vs. Santorum contest imposes different incentives on him than, say, a Romney vs. Gingrich race would have done, because the kinds of attacks that Romney would want to use against the two differ. Remember, even a sure-thing nomination such as Al Gore or George W. Bush after Iowa in 2000 still is run by people who are paid to be paranoid about the chance of losing a "safe" election, and part of what goes into an estimate that Romney is likely to win is that we assume he'll continue campaigning hard until it's over. Which means, in many cases, making new promises about public policy.
More broadly, and I haven't studied this so it's just speculative for now, I'd guess that Romney's reputation as a closet moderate would lead to more specific promises during the nomination struggle. A Santorum or a Bachmann doesn't need to prove that he or she is "really" a conservative, and so they can campaign on rhetoric alone (although in many circumstances a specific issue position can be useful if it promises to differentiate the candidates in a helpful way). Again, I'm just guessing here, but maybe that's why Romney wound up with a 59-point economic plan.
So the twists and turns of the nomination process really can matter even if they wind up with a result everyone expected from the get-go. At least, if the winner of that nomination winds up in the White House next January. Of course, that's a whole different story, for now -- except that the more contested the nomination remains, the more Romney is going to be pressed to take positions that hurt him in November. Which means that there are two good reasons to pay attention to what's happening now in New Hampshire and South Carolina, even if you're confident that Romney will wind up as the GOP nominee.