The best WHMs out there are maxxing out their regens to lower the enmity over time. Furthermore, a well-geared enmity tank will never lose hate to the WHM, not in a million years. You can think that "conceptually" DEF/VIT is better, but the reality is that what is on paper and what is in practice are different most often (and really what is on paper doesn't say much about DEF/VIT either when fighting things almost 10 levels above you like most end game mobs).