This is the pessimist in me saying that I think that Snape was a decent flier (meaning he could fly himself anywhere he chose), yet not good enough (according to Slytherin standards) to play on the house team.

I do also agree with the Snape/Lucius slasher in myself -- Lucius and Snape would *not* have been watching the game.

When I write Snape, however, the optimist wins and says that he played Keeper. I think he was too small to play a chaser and too big to play seeker and most definitely not strong enough (at the time) to play beater. Therefore, a nice, neutral position in the sky. Minus the fact that Quaffles are flying at you every which way.