I think its a case of "perception" influencing fact. Yes the legendary was nerfed but it still sims as Best In Slot for warlocks according to simcraft and Elitest Jerks. Sure its not as great but it still is nice and people can certainly come closer to you since you know they have a higher Ilevel. But the legendary can still be better then anything out there.

Class, spec, lag, play style all influence it of course.

As far as I'm aware it's still best in slot for every caster class

The legendary is still amazing. If prior to the nerf that lock was only 'eking' him out on dps, then quite frankly the lock has problems. Look at WoL for DS right now. About 60% of players in top 10 are casters, particularly fire mages. This is largely due to the legendary (also about half the melee represented are mutilate rogues, not surprising since they basically got instant 397 daggers). Keep in mind that this is still relatively low ilvls. In full heroic gear, you'll see this gap widen because the Legendary scales with every stat, which is why it was nerfed in the first place. According to current simulationcraft data casters are far ahead of other dps in BiS gear.