Most special attacks aren't fighting type though, with the only really common special fighting move thrown around being gengar's focus blast, and even then it can be defeated even by the 4x weak tyranitar (another potential special sponge) if gengar lacks LO or if ttar is +sp.DEF.
Also, in regards to Weezing (but actual in regards to dusknoir and spiritomb as well), overall Rotom just does the job better. Dual STAB, electric's welcome resistances, thunderwave and trick on top of WoW just make it much more versatile. Pain Split is an extremely overrated move, and even if rotom has it I don't think I'd make room for it.
Tyranitar and the rock type in general has really common weaknesses as is and adding the dark type just boosts it more. You'll only be able to survive two focus blasts from a timid gengar and that requires you to run a +sp. def nature and invest largely into it. I would never dream of using Ttar as a wall, while muk will still be a decent one. I'd take one common weakness over 5 or 6 anyday for a wall.
I don't think pain split's anywhere near overrated. Rotom's lack of recovery that doesn't help the opponent setup isn't cool.