It centralizes White's queen without a good way to drive it off. You can't safely use your c-pawn to do that (...c5) since that messes up your pawn structure. I've seen a lot of games lost on this site from players who made that mistake, which occurs in other openings, and with either player, as well.
http://www.chess.com/forum/view/game-analysis/scotch-game-why-is-taking-the-knight-a-bad-idea
Meant to post that here.