I currently have two tanks. One is a prot warrior, the other is a feral druid. On the warrior, I gear for mastery first, then dodge and parry. That is, I reforge off hit and expertise and gem as hard as I can for mastery. On my druid, I do the same except with agility and I reforge to dodge.
But lately, I've been seeing more and more tanks reforging to hit or expertise. Pretty much every pug raid I do on my alts will have at least one and one of our trial tanks in my guild reforged to hit.
So... either things have changed or they're just bad? I'm inclined to see them as bad. The DK tank that geared for hit and expertise was much harder to heal, even on normal Magmaw, than most DK tanks I have seen. I inspected him and he only had one piece of pvp gear (shoulders) and two dps trinkets, tank gear all reforged to hit and expertise. Just a bad player, right?