That is a viable and encouracged strategy in Goldeneye which was the first game I remember screen cheating on. Set up som remote mines and watch for people to venture near from a safe nook. It does feel cheap though. To combat this in Halo we would just lan and put the tv back to back so we could seee each others faces as we teabagged for the extra personal experience of it all but could not see the screens.
It drove me crazy when others would do this to me but how do you fight it. we are both looking at the screen and only a hairs width separated his screen from my own how can you not see where is is in a map. Now if you don't know the maps the benefit is negligible. So perhaps the answer is to have random map elements generation where all the maps have the same pieces, but they are put together in a different order when the game begins.
That would be an interesting gameply mechanic and be more realistic. You are not going to know the exact layout of a battlefield in advance. You might have a gist of it generally, but you would not know that there is a blind spot on the second floor behind a door.