As a tank, I don't really have problems getting threat even with high DPS peeps. So I put Vig on the other tank so he gets 3% dmg reduction. And if the other tank is a warrior he puts it on me? Is 3% dmg reduction not important? I see a lot of people only talking about putting it on a person w/ high DPS.
In a taunt sensitive fight this can be very bad if the other tank is not a warrior as you cap his threat and risk pulling the boss back to you. Also: Overheals are at an all time high... so the 3% reduction in damage is not really that important.