Because some people haven't played or completed it or you just want variety.
I still love raiding the vault.
Also, the reward is the battle :)
PS Touch of Malice is worse than Hardlight and that is saying something.
Sleeper Stimulant on needs one Crit and an extra shot from a sniper to take them down. Used to be two shots with sleeper and sniper at previous light levels. Not tried with Gjally with new light levels yet though. Probably hits hard as well though.
Yeah when it first came out I thought it was terrible but then I started using it against bosses and realised it is actually mega dps. Pretty much a standard for me in most raid encounters now. I'm currently waiting for a solar burn, airborne strike playlist so I can test it with Empyrean bellicose, weapons of light and tether. Ooo the humanity :)