Personally, the class/job itself should have modifiers built-in. I once suggested a PLD trait called "Shining Armor", which grants the PLD an enmity modifier to do away with the need of spacking +Enmity gear. Tanks having their modifiers built in puts the burden of holding aggro directly on the tank rather than creating doubt over whether they're stacking enough +Enmity or not. It also frees up item budget points that can be used for other stats, like Accuracy.
Yes and no. PLD's template, the WoW Prot Warrior, is built around using attacks to hold aggro. A prot warrior is either using Devastate to give the mob the stacking Defense Down debuff, Thunderclap and Shockwave for AoE aggro, Shield Slam when off cooldown and when Sword & Board procs and whatever shout they were assigned to use by the raid leader. Abilities are just for those instance where they need to recover aggro. Prot Warrior's version of Taunt is basically the current MNK's Taunt, but with abilities that supplement it's use as part of a tank's toolset. The attacks scaled part from attack power, and part from the warrior's native enmity modifiers.PLD already functions like this to a degree. MND plays a role in modifying an enmity multiplier for combos. It could be easy to add a MND based multiplier to PLD abilities and a VIT based one to WAR. I'm not against this.
Overall, I agree with the OP. It's pointless and a waste of item budget to have +Enmity and -Enmity as stats. Modifiers should be class/job-inherent.