[smittymatt]

Betting the pot removes all immediate odds if your opponent is drawing to a straight or flush (but not a combo monster draw, obviously). To draw to either a straight or flush, you need ~4 to 1 on your money on BOTH the turn and river. As you said, betting the pot gives your opponent 2 to 1 on their call, thus they are making a big mistake if they call.

The only factor that can sometimes make it correct to call a pot sized bet only on a draw are implied odds, which you probably already know of. If your opponent is pretty positive that you will pay off enough when they hit their draw to make up for the lack of immediate odds, calling the bet is OK. This is where it all comes down to player skill and the ability to get away from a hand if it is apparent that your opponent just made a better one.