I don't get why people are reading soo much into these tests from the PTR
The DK was the only one of the 3 tanks who was using cooldowns
And they were using not only their own cooldowns, but also that of their Holy Paladins / Holy Priests aswell
Yes they would have achieved more stacks than the other tanks given the nature of the fight and class mechanics
But it wouldn't have been as many as what there were, thus the tests are essentially flawed and really shouldn't be presented to bliz as a reason to make changes to the tanking classes.
The way I view it, Paladins / Warriors are there for more straight line mitigation + fast hitting bosses, DKs / Druids are there for the big hitters.
Personally I think that Bliz should stop bandying around this "You can have anyone as your MT", when the concept of even using a "Maintank" is silly and outdated. You should have 2 (Or ideally 4) maintanks imo, 1x dk/druid and 1x prot pally/warrior and the encounters should be designed around the idea that you have one of these in your raid.
Since if you try to have one particular tank MT every encounter, then you're gimping your raid because you playing to your strengths, and each encounter is skewed slightly towards one type of tank or another.