I'm curious.
Why do you consider killing multiple enemies with one shot less effective than maybe killing one enemy with one shot?
It doesn't tend to kill them. It tends to damage them IF they happen to be standing on it while it is burning... and of course they could run off it and take less damage, or if you trigger it too early they might not get damaged at all. Sure, it might be a bit AoE, but it is iffy to control, doesn't do much damage, gives away position AND obscures vision.
The alternative is a ranged sneak-attack which almost certainly will kill whatever it hits unless I'm using REALLY crappy gear... and maybe even ranged-sneak any other enemies looking for me after the first shot (since it is best done from a good sniping spot)... but in either case, I've never had any difficulty in Skyrim taking down enemies by shooting them with arrows.
So all in all.... more damage, more control. If I wanted AoE, I'd throw fireballs.