 One drawback of playing 2...c6 is that White may continue 3.Nf3, and after 3...Nf6 play 4.e3 ("Slow Slav"), and now Black cannot really play the Semi-Slav since 4...e6 is considered to be good for White as he has not committed to Nc3 thus having other options to develop, like Nbd2 or b3 for instance.Protecting the d5-pawn with the e-pawn allows Black to develop his kingside more rapidly, but it does block the light-squared bishop for the time being.