We do use Main Tank prio. For us it's more a way to make sure that the one who is considered our best tank (both on reliabilty and actually game skill) is always suited to tank the specific boss for that progression fight.
The limit of the Main Tank priority is on the brain of the tank. Maybe cause i've been offtank for long enough i know how demoralizing is when someone takes over your drops "just cause he can" and it's only a minor upgrade for him, while for you (or someone else) is a huge improvement, well... that's abusing of a priority.
Personally, i've already passed about all T7 drops, and a major part of our tanking drops.
Whenever we'll get a new Brutallus where the tank needs maximum gear, i'll consider getting full T7.25 over other people. Until then... i'll still be asking "so who wants this" in our tank channel.
Worlde - Prot Paladin
Darkworldie - Tank DW Frost DK
Uord - Prot Warrior
Huordie - Feral / Resto Druid