You hear people complain about someone camping when their team is a mess. A sniper can only kill one guy at a time-If he is a real problem (Hiding in a corner with 20 kills and 2 deaths), get one guy to distract him and have another pick him off. Basic teamwork that is almost never used. If he's not in a corner, he can be flanked if you know you're way around the map and if he's not too bright, he'll keep going back to that spot, where you can flank him again. I once got 11 kills from a sniper on Combat Arms who kept going back to the same spot.
Is it ever okay? Yes, of course it is, it's a legitimate strategy, but it does have its flaws, which can be exploited.