What bosses in WotLK that required a taunt swap where taunt immune? Gluth, Rasuvious, Horsemen, Archavon, Razorscale, Hodir, Thorim, Gormok, Deathwhisper, Suarfang. Festergut, Putricide, Sindagosa and LK where all tauntable, the only untauntable Bboss i can recall was HC LDW. I think you're being a bit rose tinted in your recolletion of wrath tank swaps, or got it confused with TBC content (I really can't recall taunt swapping on Valithria).
That said yes its a boring ass mechanic. It was a boring ass mechanic in wrath, it's still a boring ass mechanic. I think the problem is that Blizzard sees that the tank population is low, and thinks that most people don't want to try tanking becuase tanking is percieved as being hard, so won't risk making things complicated as it risks putting people off tanking.