In a good overall analysis of what's left for Edwards, Dan Balz suggests as much:

Edwards has two weeks to think about the future. He is certainly in the race through Nevada and South Carolina, the next two contests on the calendar, and at least one reason to keep going that long is that he is likely to be a help to Obama in the Palmetto State.

Obama and Clinton have competed heavily for the African American vote in South Carolina, and the Clinton campaign fears that Obama will now win the majority of that vote, perhaps a sizable majority.

Clinton's chances of winning would depend on the white vote, but as long as Edwards is running there, she would have to split that vote with him. That alone was one reason that, before Clinton unexpectedly won in New Hampshire, her advisers were seriously considering skipping the state.

Update: I now see that Zengerle beat me to this; he's skeptical of Balz's theory.

--Michael Crowley