Maelwys put it best. You cannot just see everything based on a fixed point of view. You need to factor in external factors and internal factors. The fact that going STR/DEX build makes each point healed more effective, gives you leeway in Enmity generation to Heal/Stoneskin, more power to your Inner Beast/Bloodsurge and making your Bulwark in to a more dependable CD.

Sure without external aid Vit is king. But you can't put a finger on things like snap threat, cause if you let someone pull aggro and take a hit that is already a lot of eHP/heal loss cause the healer has to heal someone else other than you that is unforeseen. Tell me the times when your Flash didn't generate enough enmity against that crit streak BLM or that time when an ambush mob came out and your healer just threw a big heal on you and had more overheal ticks incoming. How many times have you killed a mob faster and reduce that incoming damage that you can't reliably gauge it's damage? These are factors that should not be overlooked.

The fact that eHP Vit is king is true but eHP works only when there is no heal and where spike damage is involved to a certain extend. Outside that you'd want to depend on eHP/heal which is not what Vit provide. Then to top it all with more potential enmity is just an icing on the cake, don't be so tunnel visioned as to see our point is directed only to increase tank DPS.