Sleeper is a beast at raw burst damage. It is good for those bosses and such that don't leave a ton of time to dps. An example of a fight where this works out is Oryx from Regicide in the final phase. He comes out and chases you with his sword. Can you empy your full clip of spindle? No, but you can pump 2 SS lazers into his head.
Guns fit into different situations. You can even use Sleeper to take out Tanks and walkers faster by lining up the legs/treads.
English
-
Also, if you're good you can aim at a wall and still hit the boss while never getting in line of sight of said boss. Can't do that with a sniper