Tyranitar can't set up on most things you'd like to Taunt though. Ferrothorn and Trevenant can 2HKO it with STAB moves, Rotom and Tentacruel outspeed it, Forretress can Volt Switch out or hit him with Gyro Ball, etc etc. The list of viable Taunt targets is very short.
To be honest, I used the set back in DPP just to stop Skarmory from pHazing, and it worked quite well. I used it briefly now in XY and it's also did a good job. It can stop Stall teams from statusing/pHazing ttar, and that alone is a pretty big help.
And it's not true that Trev beats Ttar;
+1 252 Atk Tyranitar Stone Edge vs. 252 HP / 0 Def Trevenant: 313-370 (83.6 - 98.9%) -- guaranteed 2HKO (I do realize Crunch will OHKO, but most Trevs won't even come in on Ttar anyway, as they will fear an OHKO. And at +2 it's OHKOd by Stone Edge.)
While Horn Leech, on the other hand;
4 Atk Trevenant Horn Leech vs. 252 HP / 0 Def Tyranitar: 164-194 (40.5 - 48%) -- guaranteed 3HKO
I'm not really sure if Horn Leech (or Wood Hammer) are popular moves on Trev because I haven't really seen much besides Shadow Claw, but anyway, there you go.
The real problem with ttar is that it's weak to almost all priorities except sucker punch and Quick Attack/ES. It's x4 weak to Mach Punch which blows now that Conk's usage is up the roof and it's also weak to Bullet Punch and Aqua Jet, both which are really common.
All I'm saying is, you don't miss out on much except for 100% accuracy when you forego Crunch for Taunt. Gengar is still OHKO'd by a +0/+1 Stone Edge.