In endgame content a group of mobs can take you down quick if their fire is focused on you. AoE grenades are great for dispersing mobs as well as majors. They can effectively blockade enemies from approaching you as well as trapping them in place for a couple of seconds. If there aren't majors around its really a waste to have sticky grenades.
1. you shouldn't ever be in a situation where you're surrounded by mobs.
2. If you're in said situation, then sticking an enemy with a grenade will cause all nearby enemies to duck for cover, giving you time to get away or just pop them all in the head.
The main point is mobs die in like 1-2 shots anyways. Sticky grenades offer the burst power (almost as much as a rocket) to take down the hard stuff.
While you're "dispersing" your mobs I've already killed them all with 5 quick handcannon headshots.
But see while I'm dispersing those mobs I'm killing most of them with my grenade and can then use my handcannon to kill the remaining mob(s) with less than those 5 shots. Sticky grenades may be effective for single enemies but AoE grenades are more effective for mobs. This is like arguing about Word of Crota vs Fatebringer essentially.