I disagree. If you're going to have pistols, revolvers, assault rifles, machine guns, shot guns, etc....it only makes sense to add Sniper Rifles as well. It's a weapon, it's very much used in combat/warfare, and it has it's advantages and disadvantages like any other weapon. The problem isn't that developesr add snipers to games...it's that they make them way too easy to use. Sniping is extremely difficult ESPECIALLY when the target is moving. Developers just have to figure out how to incorporate them into games to make it a more balanced experience.
"...it's very much used in combat/warfare"
Yes, but at what ranges? you don't see snipers making pot shots at 50m. If the enemy got that close, a real sniper would get the hell out of there and find a new position. Games often include snipers as a weapon in games, but very few games include the role. Firing a 10kg rifle with precision accuracy while standing? good luck. A scope that magically adjusts itself for wind, gravity, spin drift, temperature, and altitude? If they actually included all of the cons that snipers have, no one would play as one.