I can't answer why it's a thing, but having run most of the trials at lvl 50 (Can't speak for the lvl 60 dungeons) one or two months ago I can answer why a tank might do this. I remember in The Chrysalis there is a point where the OT can grab threat to help mitigate damage to the MT, and I don't know how high your health is but our MT died from the three shot thunder attack (or whatever it is) that the boss used.

I know that as a WAR I ran around only using Defiance until I got Fell Cleave because Deliverance was useless to me beyond a slight bonus damage that Maim could just as easily offer.

In the case of running with my PLD, I usually ran in Shield Oath because I didn't know what was going to happen. Some tanks are great at what they do others could screw up and if I'm in Sword Oath I might not be able to take the hit resulting in a wipe.

These are just my thoughts as one who only knew the fights through what I saw on videos. Maybe they can offer some insight.