MCH main here. It depends on class really. AoE ain't that great on MCH to start with.

My rotation on mob pulls mainly consists of Bioblaster, Flamethrower, Ricochet and Auto crossbow. Bioblaster is up basically every 20 seconds. I always keep that up. Flamethrower you can only use once per mob pull because the pulls will be dead before the 1 minute timer. Ricochet i spam in between and Crossbow i use when my heat gauge is up.

However on the last mob pull or split between both i do go through single target rotations to get my battery gauge up all the way and heat gauge. This is mainly done when everything is on c/d or i weave them in between AoE's. It's so i can start all the bosses with everything up and i can burst it in the beginning.

I don't bother with spreadshot because most pulls done are dead within 30 seconds. And depending on how big the pull is my single target rotations will deal more DMG than AoE overall. (Usually 3 mobs or less).

OP states " Explain to DPS that on about 3 targets, it'd be better to do AOEs". This is wrong. It's better to do single target rotations on very small pulls consisting of 3 or less because you can burst down them faster then using some weak AoE skills. Single target rotations of course have higher DMG potency than AoE.