There are some l4d players using snipers as shotguns, whereas they are completely different weapons and the role of their user is completely different too.
A good sniper doesn’t care about score at the end of each level; a good snipers cares to cover the teammates from specials and commons behind them; a good sniper uses the zoom to alert the teammates about dangers ahead; a good sniper never makes friendly fire, cuz the role of protecting teammates does not allow it; a good sniper does not burn tanks and run away to shoot them from safe place whereas teammates are facing the beast with shotguns risking more due to the higher speed of the beast (on the other hand, burned tanks have shorter life, so is a waste of the best toy of the game).
In conclusion: a good sniper is a good teammate, not a selfish high score hunter. I only know 2 good snipers, and both are girls (perhaps they know better about care).