Fire i is faster so if it's a quick kill trash pull, it can be faster to single shot. Let's say I target something and cast fire ii.... and now it's dead before the casting finishes. I tab to the next target and fire ii... and that one's dead before it finishes. In those cases, I single shot those.

Adds on a boss, sometimes that's single shot because I'm low on mp and don't have the time to transpose and blizzard but I can land a single shot.

I'd be all aoe all the time on 3 or more if I could, but there are times singles are more effective.