Personally when I Q as tank for Trials and the like... honestly a lot of time I'd rather MT than OT (provided I'm comfortable with the fight obviously, if I'm not then I am likely Qing for it as healer instead).
1). OT is boring as balls, you just do the highest damage rotation over and over again and... you rarely do anything else for the majority of the Trials other than occasionally pick up an add or two that will die 5 seconds later.
2). It's a lot more running around and grabbing adds/objects/what-not and that's just.... eeew, I'm not a fan of it.
3). I end up not trusting the average other tank in Trials, and I'd rather just do it myself because I know I'd be better at it.
4). Half of the time they don't assert who is MT and who is OT and they will either fight me for aggro, or only half do their job if I let them MT.
#4 is what annoys me the most... I go in and watch to see what stance they throw up. No stance. awesome. Sometimes we wait and wait until I go 'screw it I'll do it myself' or the tank will finally throw tank stance on and go attack the boss.
One of two things happens... the tank will actually MT, or they'll haphazardly dump abilities on it and their threat generation blows and I'll rip it off them while in DPS stance. Then my own tank stance goes up.
Or, in the case that we stand around doing nothing and I decide 'screw it, I'll do it' ... the other tank will throw tank stance and fight me for aggro. Then I gotta go to DPS, and try to find a way to get off #1 threat so we don't end up cleaving the raid or something.
Just.......nngh.
Seriously. Pick one. OT, or MT. Pick one, and assert. I'll adjust. But I hate guessing which the other tank is gonna do, because yanno, 90% of the time, the other tank ain't gonna reply to anything I say in chat. There was a time I used to ask before the pull, but I fell out of the practice because 90% of the time, I never got a reply.