People will bitch if you're beside a weapon or item, but that's their problem. They've got to come up with their own strategy. Well, I can play that game as well. However, the primary requirement is you're constantly using the railgun, even if you have the BFG and full ammo. However, they could if they wanted to.
Camping prevails over sniping because it's exponentially less annoying.
It's better to have a guy sitting behind a pillar or next to the rocket launcher than to have a guy way out on a platform with the rail. Campers, on the other hand, typically do their work by denying everyone access to a crucial weapon or powerup. As for your looser definition, hiding anywhere is just unnatural. Okay, let's take a specific example, and this is the one where sniping hurts the most. Q3DM17, The Longest Yard.