It is insanely useful.
If you have an option to grey out the tiles your operative / squad has no vision of. Vision=can detect enemy in that tile.
It is awlays a PITA in such games to understand where you can and where you cannot see, since this often costs a life of your operative when one single tile is left unnoticed and enemy from that tile can counterattack when you are very close to him (most evil for me in JA2, where death of your merc is much more serious loss than that of X-Com operative, and sometimes its just insanely hard to understand wether you see every tile where enemy can possibly be or not)
By having a clear understanding which tile does an operative see and which he does not, playing this game will get lot less frustrating and allow you to focus more on thinking rather than calculating tiles by hand... multiple times... for multiple tiles and multiple operatives.
Also after researching certain alien X-COM operatives could incorporate that data to be able to see alien's sight range (like in JA2 1.13 you can see your "danger zone" which equals to enemy sight range, adjusted to your stealthiness, camo and land cover). This could help players study the way to play against aliens, study dead zones like those pesky corner ambushes, learn about how to generally approach aliens and make door ambushes and so on with in-combat on-demand help.
Also we could add a very stylish "Motion Scanner 3D view" or "Motion Detection Headgear" or something, where using the item would turn everything to dark gray and tiles where movement was detected would be transitioned to bright yellow, depending on the amount of movement done.
Maybe also make an additional research option unlocked after having researched several alien corpses and probably live aliens - Alien Lifesigns Scanner - which would again turn everything to dark gray and nearby tiles where aliens are would be tinted in a color which signifies a kind of an alien (machine, coldblooded, humanoid, etc) is there - possibly with a chance of a false positive or with a chance to show PREVIOUS alien position based on agent's reactions (meaning, imagining this all actually goes in real time, agent was a slowpoke and since the time he managed to scan the alien location, alien already moved itself). Or maybe it could show trace of alien movement, with more color = more recent position and less color = position further back in time, this would make reading the scanner's output harder and at the same time would not need to "break" it in any way by false readings to prevent it going too imbalanced good, on the other hand allowing players to study alien behavior from playing the game, not from reading WIKI articles on AI and looking at waypoint maps. Would make sense too, as often seen by special ops in movies (i suppose they take it from RL, not imagine) where they use thermal and other kind of scanners to check the movement patterns of the terrorists before making an attack, to minimise casualties.