So after a few lucky pugs I managed to grab the helm and and pants of the Erebor Leader (tanking) set.
First I'd like to point out that the Defensive Strike proc (+1700ish tact mit [estimated value from memory]) does not show up on the tooltip (tooltip looks the same as w/o the armor) so it is difficult to guess what percent chance this proc can occur. I bug reported this ingame.
Now from my own tests stabbing the training dummys with my tanking spear I would wager that the proc happens ~10% of the time; not terrible but not reliable either. So basically spamming Def Strike which I do when tanking anyways I can assume that: every 3s, I have ~10% chance to raise my tact mit ~2% for 10s, tested it in my full tank set up which has 62% tact mit with out scrolls. In a raid where I was tanking, the buff put me just under the tact mit cap (~69.7%).
So I ask, why bother? such a low chance for a arguably lackluster buff that isnt dependable. Personally the only reason Ive justified keeping those two pieces is because the only other set bonuses that are useful with tanking are the Hytbold charge for more Shadow Lamnet damage/threat or the ToO tank set for a marginally more reliable power restore with Def Strike.