I make this post with some slight hesitation due to balancing issues that may arise --
Realistically, Sniper Rifles are second only to full-automatic heavy weaponry as far as suppression goes - especially in terms of military combat. The average untrained person might get scared stiff from the roar of a shotgun, but trained personnel seem to handle it better. On the other than hand, when it comes to getting shot at by a sniper, it's full-on panic mode and everyone scrambles ( or maybe that's just too many war movies ).
Their damage is incredibly high ( obviously ), their accuracy is outstanding, and they come in an extremely wide variety. For example, some of them are meant to be mobile (in terms of sniper rifle weight, at least) and fire smaller caliber rounds, while other sniper rifles are heavy, hulking behemoths that are meant to be set up at extreme ranges and not move very often.
As it stands, Realism vs Balance is precarious when it comes to Sniper Rifles in particular. Even now, in the vanilla version of the game ( both on the normal and Experimental ) I run with 3 Snipers. With mods ( namely: Little Mod ) I use 4 and they absolutely destroy everything that they have line of sight to - up until we get to the tougher enemies. It's hard to balance them - too much damage, armor penetration, range, accuracy, and suppression makes them good for everything -except- going into the downed UFO itself.
By the same token, later enemies mitigate a lot of their usefulness ( aka; the androids ), who shouldn't be suppressed -ever-, and whose armor makes them even take 4ish rounds from a standard ballistic Sniper Rifle and even more for any other kind of ballistic weaponry. Even more so, larger UFO's mean more of the combat takes place inside the UFO itself, which yet again severely limits them.