I think this is literally the worst thing you could do. At the end of the day dps is more useful than mitigation on tanks. Also utility needs to be defined because dps utility is again far more useful than mitigation utility. I realize this sounds paradoxical but I will explain by copy and pasting a reply I had made in another thread on the same subject.
"Okay let me break it down. Mitigation at the end of the day for a tank is fairly binary. You either have enough mitigation to survive what you are tanking or you don't. It does not have an infinite cap in utility the way damage does (infinite is a clear exaggeration as dps limits are set by the amount of hp the enemy has but on a practical level dps cap is infinite). In simple terms, more dps is always useful while mitigation has a hard drop off in use. Why does this cause a problem? If all tanks are able to survive an encounter there is no purpose to being a "mitigation specialized" tank. Everyone will min/max their groups with "dps tanks". If an encounter is designed so that only the mitigation based tank can survive than we have a very problematic situation where some classes designed as tanks (the dps tanks) aren't even able to perform their basic role of being a tank. More often than not the first scenario is going to occur and "mitigation tanks" get the short end of the stick and get excluded from groups. Or Mitigation tanks are more useful as progression occurs and then once people understand the encounters or become more geared the mitigation based tanks are again excluded from groups.
Now one can argue that the mitigation based tank can be designated as the main tank and the dps tanks can be designated as the off tanks but this would again only hold if the encounter is designed such that there is so much damage that the dps tanks simply can't fulfill the role of the main tank.
One could finally argue that the mitigation tank is able to take so much less dmg that the healers are than able to dps that much more to make up the dps loss of having a mitigation tank. This results in far more difficult balancing act because now balance is now going involving interdependence of classes."