Let's do a little thought experiment.
I'm private Fritz (or Ivan). I've got this huge freaking tank of very nasty stuff on my back and I'm trying to do my damnest to avoid even being seen, much less getting shot at.
My sergeant, passing on orders from the Great Player In The Sky, orders me to torch the next door building. Doing this will of course will require me to stand or at least expose part of myself, and when I light up every bad guy, who all by nature hate flamethrower carrying soldiers, will be trying to kill me.
So what do I do? Do I carefully observe how much fire I lay on the building, stopping when it seems sufficiently alight? And how do I judge that, especially when (and this is something probably not considered by many) while the flamer is active, I can't even see my own point of aim?. To accurately observe my efforts, I should sent a short burst, look, send another burst, look, etc.
WHAT, AM I CRAZY?
I do that and I give every enemy nearby plenty of time to hunt me down. So instead I give it several big honking blasts, trying to not be too obvious that I am deperate to use up all the fuel so I can drop the whole ensemble off my back.
So I agree with The_Capt, no reasonably self-preserving flamethrower wielding soul should be counted on for more than one target. If you get more, great, but I think that CMBB's TacAI correctly has the guy try to use up the fuel right away.