I've never had threat issues to have any use for inquisition as a tank. The boost to SotR for single target threat is very small and won't save you from dps blowing their load as you pull which is the only time you should even possibly have single target threat issues if you're paying attention. The AE boost is more noticable but I still just can't be bothered with Inquisition because I don't run with bad dps to need the extra AE threat.
Yes, I'm one of the tanks that queues random as dps for points to buy tradeskill stuff I can't be bothered to farm with my alts and only tanks for guildies and friends.