Dark Simulwhatever will certainly have its uses in PvE. While it can't be used to reflect damage, copying it is pretty darn neat too. Can't see why tanks wouldn't be able to use it, or DPS.

As for taking the spot of something else: do keep in mind that they're redoing the rune system and our strikes (iirc, they even wanted to get rid of death runes at some point, but I might be remembering wrong), so I'm fairly sure there will be plenty of opportunities to use DarkSim, if the fight allows, without gimping your threat/damage/survival/whatever.

Just as an example: copying Jaraxxus' fireball would be neat: AMS the hit, copy the spell. That hurts quite a bit! Or Deathwhisper's frost bolt (which you can pretty much soak up in frost res gear anyway)... there's a lot of other examples where DarkSim would be handy.