Gas sniper rifles require no upper body strength to cock the bolt. A high-tensioned spring is often quite difficult to cock; you almost certainly lose your sight picture as you have to really work to cock the bolt. With a gas sniper rifle (like my Tanaka M700 AICS) you don't even have to move the rifle to re-cock the bolt - keep your eyes looking through the scope and flick your wrist.
Most spring sniper rifles are neither particularly accurate or consistent right out of the box without a couple of hundred dollar in tuning and modifications (tightbore, Zero Trigger or sear set). The same can be said for *quality* gas sniper rifles. Add a few parts and they can become very consistent.
There's a great thread here about Illusion's experiments with upgrading his gas-powered Tanaka: