I disagree. This rule might apply to real life, but definately not in game. On some maps, especially Muttrah, there are fixed optimal sniping points where you seldom move away from.
If you want to be a dedicated recon guy, take an officer's kit instead. Not only that, recon works only if a few factors fall into place:
1. CAS listens to you or you are the SL of the CAS squad who can mark out the targets
2. There is an actual commander who can mark out targets on the map when you send them in
Snipers can & do hold back entire squads both ingame and in real life. If you get sniped at in real life, no one dares to go out. The value of the fear created by a sniper is so great that commanders are willing to call in an arty on a jungle patch just to clear out the threat.
Ingame, you can hold back a squad for some time while they try to hunt for you. This means that your team can cover more ground in the meantime. Best case scenario is if you snipe the whole squad of course.
Solo sniping has its justifications. As mentioned by earlier posters, team mates although useful, tend to give away your position more often than not. Also now that medic kits are limited, bringing 1 as an insurance policy and spotter might actually detrimental to the whole team.
A sniper is useful & in certain scenarios can turn the tide, however the real work is still accomplished by the straight legs on the ground. So the more players there are in a sniper squad, it is not really useful as there will be less players to help in the regular squads. Hence its is understandable when snipers solo or just duo. I really detest when players sprout the rule that "Oh snipers are better in a squad". This is actually not true for every player.
Snipers should never be in a 6 man unless the server has very few players. That is why I never begrudge someone if they take the sniper kit and leave. A 6 man doesn't need the kit, a sniper wastes 1 spot better given to another kit & a sniper is better as an SL in his own squad anyway since he can call in jdams.
There shouldn't be 2 snipers roaming together as well. You risk losing 2 kits at once, is easier to spot and have less coverage of the battlefield if both of them camp in the same spot.
There's some other tips, mostly for Muttrah, which I want to write about, but that's it for now I guess.