It's a legitimate question. I'm still a n00b so there are nuances I'm not aware, but look slike the flamethrower has basically no purpose.
I only use it for two reasons. One is because according to the wiki aliens on fire regenerate energy at a severely reduced rate. This means that if you hit an onos, he'll be out of energy after stomping at most twice and be basically defenseless. Since onos in general are currently what they are, this is a good reason to want one jetpacking around, though when/if stomping is adjusted or marines get other means of dealing with them this reason will surely dissapear.
(the other reason is because I have clear feedback on whenever I hit. Hey, the critter's burning, this means I'm not so useless at firing. After emptying two rifle clips on a skulk without knowing if I hit once, it's a nice change of pace)
Other than that, it costs 30 freaking pres, has a really short range and deals little damage. I would understand it's usefulness if it did increased damage to buildings, or if alien buildings set on fire didn't work, or if it cleared infestation, but it doesn't look like it's the case. What does the flmethrower do or is supposed to do that it doesn't that justifies not only it costing 30 pres, but not choosing another weapon?