Remember, there's a difference between "easy to heal" and "least amount of healing required".
KBN and dipstik's numbers are focused upon maximizing mitigation, which means reducing incoming damage as much as possible. It's a long term view.
By stacking Shield and getting rid of Defense/Absorb, you're making it easier to heal you (by reducing spikiness, which can best be described, in this sense, as "chance of an unmitigated hit going through") but increasing the amount of damage you take and healing you require.
The reason that mean mitigation means less than it used to is because, right now, tanks aren't dying because they're taking a crapton of damage over the course of a fight or across a large number of attacks; they're dying because they're taking a *lot* of damage over a short period of time. As such, survivability isn't so much a question of taking *less* damage but taking damage in a manner that can be healed in a more steady manner.
And that is exactly why I use the proc shield relic instead of fortunate redoubt on my pt, even though redoubt is BiS, I may take more dmg but I think that when you trust your healers enough (if you don't you want to take less dmg probably) to work more on reducing spikiness than overall dmg taken. A lot of incoming dmg is no problem for a good healer but very spiky and unpredictable dmg can make even a very good healer let his tank die.
Also the reason why I have the immunity and sturdiness enhancements on my shadow, and I'm not switching until I get good 31 enhancements to replace them (which pretty much give the same mitigation but better endurance, in other words I wont be less spiky just survive a little more)
Those tank distributions and pretty much everything on theorycrafting is a guide but its never something you should follow blindly, you have to understand it and adapt it to your playstyle and situation, another very good example of that is that I always have spare relics on inventory so I can adapt to the boss I'm fighting, like dipstik pointed a few posts before dmg profiles make a difference in the stats distribution and this is even more noticeable on shadows, while its not really viable to have different gear sets for each instance or fight its always easy to swap relics.