I think one of the problems that we keep running into is that you can't balance support functions (i.e. mitigation, utility, healing) against dps. One of the reasons why developers tend to be very cautious with mitigation is because it can very easily get out of control without being carefully held in check. In this game, for example, there are many mechanics which are triggered by the player taking damage (i.e. debuffs, knockbacks, etc.) If you manage to completely negate the damage, then it effect doesn't occur. This allows you to bypass mechanics which you were originally intended to deal with.
In order to prevent you from reaching 100% mitigation, many of the buffs/debuffs use multiplicative stacking, so you get diminishing returns for each additional buff/debuff. There are ways around this, however. Hallowed Ground completely negates damage on its own. Flat damage shields, such as PLD's DV and healer shields, also allow you to completely negate damage. As long the devs continue to design mechanics which require damage in order to trigger, these types of moves run the risk of allowing players to exploit weaknesses in fight design. I see this as problematic as it is, and am grateful that what little checks and balances we have do exist. Buffing DV's shield strength is an extremely bad idea, especially when it uses flat DR instead of %DR, unlike the other two tanks' raid mitigation tools.
It's easy to point to the dps discrepancy on fights in which you can find workarounds for not having access to the extra utility. But if you can't find a workaround, then you make that one tank mandatory because they're the only one able to cheese the encounter. PLD's kit may be bloated with "situational" utility skills, but if you keep piling them on, you will surely find a way to break job balance. (PvP is a fairly good example of how PLD's "situational" kit can come together to become incredibly powerful, in the right circumstances. DRK is relatively rare in that environment due to a comparative lack of utility.)
This is the reason why we're seeing recurring issues with tank and healer balance. Balance dps against dps, and utility against utility.