See topic title, I always assumed Brackenshell were BiS
I know absolutely nothing about protadins, but for DK tanks dodge rating is pretty craptastic. Also STR now give parry in MASSIVE quantities. So Backbreakers give more STR (parry and dps/threat), mastery which for a lot of tanks is best secondary stat, and more stamina.
Would have to do some math to see which is actually BiS nowadays, but I can see why Backbreakers might be it.
Because back before the patch, the mastery gave us more CTC due to mastery being such a winning stat. By the time you would get those shoulders however, you would be attempting to keep your CTC down by gemming stamina - those shoulders have more stamina than Brackenshell and 2 gem slots, allowing you to get more stamina out of them and your gear in general, than Brackenshell.
The reason I used them was for more DPS, I used to do some Prot DPS at the end of DS and now just haven't bothered to change them (not sure I'd really want to due to the mastery and gem slots). Your reasoning is 100% true, though. Just wanted to point another reason out.
Pre-5.0.4, those spaulders might not be ideal for tanking, but the mastery makes them at least something to use if the Brackenshell Spaulders refuse to drop. You can always reforge the crit into dodge or parry as well.
Putin khuliyo
It was actually the other way round. Pre 5.0.4, you took these shoulders if they dropped It wasn't a case of them not being ideal, they were brilliant in the same way that Hagara belt > Blackhorn belt for tanking, because you'd be trying to stamina stack and due to the mastery and in the spaulders case, the ilvl, the extra stamina was worth the dps stat.