Let me ask you this: If you were a Queen's pawn player, and some one played 1. d4 d5 2. c4 Nf6, would you not take the pawn on d5? Would you not deduce that 2...e6, 2...c6, and 2...dxc4 were the best moves because the dealt with the tension in the center in the best ways? It's basic positional chess that you want more pawns in the center than your opponent. As far as Jonathon's post goes, taking the pawn HAS to be the best move. Your opponent is giving you a central majority and a pawn. 3. d4 allows your opponent to weasel his way back into a Slav. Maybe he wouldn't want to play that, but 3. dxc6 IS the best move. The best move against the Morra is to clearly accept the pawn, and it's the same here, except here Black will get nearly nothing, since he's a tempo down. It's too easy for White to deal with the ...e4 break if he has another move.
EDIT, and digression: It seems to me that a lot of players lack a principality about the game. No one cares about precision or trying to find the very best move, or prove something over the board. They just want to play what they're used to, instead of playing the best move. I can't help but feel a little bit of dismay towards this, as I feel it's a crime against the game. Maybe it's a fault of mine. In some ways, this tendency can be excused a little bit in the opening, because no one can be sure what line is best, and it's very much a personal choice, but I think it's a pretty clear cut case here that White should take the pawn and just hold on to it. There's a reason no one plays this way, a reason why Black doesn't play 1...d5 at all, even if he takes with the queen, etc....Black can't play this way in a game of chess and expect a good position.