Nate Silver says Mitt Romney:
Although Mr. Huntsman had relatively little support in the polls outside of New Hampshire, recent surveys suggested that the plurality of his supporters had Mr. Romney as their second choice.
[…]
Because Mr. Huntsman had relatively little support in South Carolina — between 1 and 6 percent of the vote in recent surveys — the net effect of this might be fairly negligible in the immediate term, with Mr. Romney gaining perhaps a percentage point in the state relative to more conservative candidates like Newt Gingrich.
Still, and particularly with Mr. Huntsman primed to endorse him, this removes another obstacle from Mr. Romney’s path to the nomination.
And while Huntsman’s support is small (hence the drop out), every vote helps.
Silver provides the following pie chart of second preferences for Huntsman’s voters:










