Allowing a tank job to exceed other job's total possible hate is similar to giving a healer infinite MP.
Exactly. Then things would be balanced again ;p

But seriously, there many things that allow enmity to change in a fight, such as monster abilities that reset hate and damage taken by the tank. So a higher enmity cap doesn't ensure the mob stays adhered to the tank, nor does it ensure said tank can stay alive. But it does allow DDs to be a bit more free flowing with their damage which is really the point of getting the best gear. If you have to intentionally gimp yourself to maintain hate on the tank, then what's the point of great gear? Look at me I have superior gear but I can only shoot an arrow every 30 sec or I'm dead. I'm so much better than the guy in average gear that can shoot continuously for just as much damage.