It’s subjective so not every tank will agree, but we believe it is more fun to hold threat because of dealing damage than to hold threat through other mechanics. Granted there are probably tanks who would be content to do 20 dps, so long as they could hold aggro. When you’re dealing with an audience this large, you can’t limit yourself to “Would *someone* like it this way?” You have to do what is right for the game overall. (Source)

The real issue is that Vengance doesn’t do a thing when you’re hitting your buttons. Vengance, in its current form, has nothing to do with what you do. It only happens when you get hit, counterintuitive, reactive mechanic instead of an active one.
Vengeance is an attack power boost. If you aren’t maximizing that attack power by hitting the right buttons, then you aren’t making effective use of the mechanic. Your argument is like saying that Blessing of Might isn’t an attack power boost because it ‘s caused by something else (the paladin) and not by what you do. (Source)