The thing is that there aren't many sources of extra armor before 251/564 gear (with some exeptions and the cloak enchant). Of course armor matters, but the armor difference on gear is so low that it's hardly the thing to take into consideration while choosing an upgrade (again before t10/frost emblem gear with extra armor on it). And if the tank is raiding anything apropriate to his gear level then stamina clearly is in most cases the best thing to gem. An believe it or not, no tank is going to regem his gear with avoidance to do the weekly raid. FWY brewfest trinket is the second best ilevel 200 tank trinket.