Your hilt/barrel adds to your Force/Tech Power so it matters alot for bonus damage/healing.
Your MH weapon adds to both white damage, by increasing your basic weapon damage, and your yellow damage, by increasing Force Power. As a tank, since you shouldn't be stacking power, surge, or any of those DPS stats (with the exception of the power that comes automatically with using tank augs), you only get increased damage from 2 sources: your primary stat and your base weapon damage, each of which account for roughly 50% of your total damage dealt. Your primary stat is pretty evenly distributed across your gear, but your weapon damage is based *exclusively* off of your hilt. As such, the hilt is the single most important slot from a perspective of tank damage/threat.
However, it won't really cripple you as much as you think. If you're having threat generation problems, just start spamming the living bejeebus out of your taunts (you shouldn't really have had any problem on the boss you mentioned since you should be taunting after each threat drop and gaining a pretty decent advantage with each one). The only time you *can't* do this is in a fight where you have to explicitly reserve your taunts for explicit tank swap mechanics, none of which happen in KP or EV.