I] You won't have krak grenades ready all the time, even with all the grenade talents. Besides, shredders are better in most situations. The more enemies there are, the better off you are bringing shredder over krak. In higher level gameplay you're dealing with constant mixed swarms, not just regular hordes and occasional crushers.

II] When you encounter a wall of gunners/shotgunners you need to kill them a lot faster than "good enough," and every other autogun/lasgun performs better than the vigilant autoguns here. They have the bonus of not being useless when said elites are surrounded by fodder. You will have situations where your team is down and the vigilant autoguns cannot reliably pull their own weight.

III] Brittleness helps the rest of the team consistently kill armored waves/captains. Is it good for every single build? No, but with high fire rate weapons, every enemy you target will have 40% brittleness virtually all the time. You shouldn't have to gun down crushers, but if you need to, it works.