(1) If nobody with spare TU and a line of fire can see a soldier's back, that soldier is probably going to die.
(2) If you don't have the aliens completely encircled, they will be almost supernaturally good at hiding from you. If I read the code correctly, they actually cheat a little bit to find perfect hiding places. Aliens almost always avoid ending their turns in visible locations.
(3) Therefore, a good mission plan (at least for a relatively open map) is:
(a) Deploy snipers to high ground where possible; this holds the aliens in place and puts them on the defensive.
1. very wise
2. The aliens actually do cheat, in that they always know where you are [regardless of whether they can see you].
3. There is a problem here in that Line Of Sight is not the same as Line Of Fire. A sniper on a tall building will have LOS to most outdoor points on the map. However, most such buildings in UFO have a parapet or lip around the building that blocks LOF. LOF currently emanates from the hip, though there have been discussions about moving it higher. In fact, Nemchenk was looking into using current LOS code for all firemodes other than snapshot but i don't know how far that has gotten. The other thing that reduces the value of snipers is that by the time they have climbed to that high point, the aliens have already all gone indoors.