I personally don't rely on proc or on-use trinkets. I just go with stat-stacking trinkets Fel-Spring Coil (with a gem socket) and Wrath of Unchaining from Spine. Barrage will max out stacks on Wrath instantly. Granted I go BM for TWing since most of it is aoe.
Sure, Mirror is an amazing trinket, but I'd rather not assume the pug tank is going to keep the mobs in it. Plus, it won't be up for the next 3 or so pulls anyways. With DBW it would be up for one pull, in between pulls, and then down for the next 3 pulls. They just seemed more frustrating than they were worth.
I'd suggest people pick the trinkets that make sense for them. If your guildie tank does the dungeon in 3 pulls, Mirror all the way.