I'm sorry but you are talking shits.
How are you going to extend anything if server is sending you packet with info when you see something or not ?
Everything is server side. It's checking if you are close enought to someone, it's checking your stuff and then it's calculating chance for spot etc.
You can't see something what you don't even know it exists. That's the idea of well coded mmorpgs. Client is waiting for info from server, everything like experience, view range, radio, aiming and more is server side. If it would be client sided there would be no chance to keep playing fair for everyone cause even with lags you could get some extra abilities. For example driving your tank: It's created on the map, and as a commander of your tank You can give orders to change his coords x,y,z, his turret x,y,z, and gun x,y,z, to fix something etc.. Everything like this is based on vectors and checks and server will not let you change your for example gun coords to -x,-y,z in a second cause he will check how fast it is possible and it will give you info to your client about every little movement.