Good snipers: Contribute to the game, by picking off any enemy in their sights from a concealed point with a good view of the war zone, often saving team mates lives or stopping enemies from reinforcing choke points and getting an equal amount if not more kills than someone on the front lines.
Bad Snipers: DON'T contribute to the game, by taking up a vital team spot that could be used for someone useful. Often take up useless spots or stand blatantly in the open asking to be killed by someone not even trying to snipe, like someone with an assault rifle saying "oh, look, a shit sniper *fires a quick burst kills the sniper*" and the 'snipers' team gets a helping of verbal diarrhea usually "OMFG HOW DID HE SEE ME I WAS IN COVER!!!"
Personally when I'm playing BC2 I have a preference for sniping, but won't if there's already half the team made up of snipers getting zero kills and wasting team space. Otherwise I'll try and play the 'mid range game' and do my bit. Also, the appeal is arguing with your friend over who kills someone half the map away, like they are some insignificant target on the Kilometer long firing range, then when you both take the shot, hit the target then argue over why the other person took the shot when you clearly stated it was yours. Alternatively working with your friend when sniping, calling a target when the bullet is an inch from his face, on the off chance you miss for your friend to kill him. That is efficient sniping. Between the two of us we usually rack up over 50 kills.