Has anyone found an actual good use for flak cannon?

The Vanquisher supersedes it; otherwise we would see more Flak Cannon nuking of spawns

Try this. Equip the Falcon, fly above a spawn of health mobs or non-boss non-Berserker armor, and Flak + Falcon away. Then if you're the Merc, drop down and Flame. Rekt

See here from 1:08 

When I head down that ramp with the Adhi and the Raider I could have bombed them with more and pretty much instantly kill them, but, not wanting to run to ammo boxes, have been holding back lately.  Which I will endeavor to rectify, just for the sake of having Flak Cannon clips

You need to have a good angle to bounce the flak the right way. An easy way to see the damage it can do in confined spaces is to start a Gold Kett solo on zero, find a corner where they rush you, and Flak, Falcon & Flame