On Warpath, I can understand a few snipers at certain stages, as well as on Convoy. But sometimes, there's absolutely no way a sniper can be effective. Sometimes the battle area is too small, or has too many tight corners, or there's too much going on for a sniper to survive the seconds it takes even a good sniper to line up (and charge) a shot. The biggest problem is when there just aren't enough enemies (or snipable ones) running around to make it worth while, like a 4v4 battle on 2fort, someone will go sniper and stand uselessly on the battlements.
However, I've seen a few good snipers make a difference, especially on Warpath and on defense in the first cap on any Dustbowl stage. When backed into a corner on Warpath, at the last cap point, a good sniper can make a huge difference. I've went sniper there on defense and gotten 100+ points just in my time at that cap, about 20 per life (interrupted by the occasional spy that couldn't resist). So yes, there aren't always too many snipers.
Still, it'd help if they went Medic or Engie sometimes. Especially the bad ones that won't improve. ;P