For other tanks maybe, but for Blood DK, a lot of their mitigation is in fact based on max health. A lot of their mitigation are after the fact heals, and many of them say "X% of damage taken over the last A seconds or Y% of you max health, whichever is greater". So, if a Blood DK stacks enough max health it makes their mitigation A) bigger and B) more predictable.
You raise a good point. It's poor design that the paladin actually need to be in melee range to activate this, actually. It should function like the Captain America shield, since there are times when you can't maintain contact with the boss and need the mitigation. It doesn't counter the point I was making; however, it does serve to support the superiority of a bear over (any) other tanks.
I can teach you how to play, but I can't fix stupid.