A flamer has such a high rof and such a high ignition chance that you will have a fire on a component again once the immunity runs of. Thus rendering you to constantly camp a component which will stop you from doing anything else until you are out of range for the flamer.
Exactly why you need the chem spray. Using the extinguisher to keep a single important component from burning down means you cannot turn your attention to anything else.
Thing is, in many situations it's not that big of a bother, like on a balloon side of a pyramidion for example. You can quite often focus on extinguishing the balloon without any problem until enemy armor's down, then mount the mortar, kill them, that way dealing with future flamer problems. On a junker, for example, top deck engineer can spray balloon, side engines AND the hull, while front engie can keep the fire extinguisher in times of need, like in case of lower deck burning. Hull is sprayed either way, but you are also prepared for other threats.
The only way is perfect chem spray. Damned if you miss a chem spray, just as dammed if you futilely attempt to extinguish fires all over the ship, only to have another 10 stacks on the first component you extinguished.
That's the thing, even perfect spraying is not enough anymore. Let's say you're fighting against gat/mortar and carro/flamer pyras. At one point you'll be left with a choice, either protect yourself from enemy mortar by using a mallet on the hull and keeping you alive, or protect yourself from the incoming flamer by re-spraying before the previous hull freeze runs off. Either way you're screwed, but incoming flamestacks rising on your hull can be dealt with quickly by an engineer from the balloon with fire ext after mallet cooldown is over. Of course, whether or not it'll be effective depends on how long the previous chemspray freeze will hold on the hull, will flamestacks deal more damage during up to 9 seconds than what you managed to repair with a mallet, but that way you dealt with gatling threat, and in most of the cases the intervention of a second engineer will save you from the flamestacks as well.
This actually has even been abused by Bubbles himself on one of the DevMatches after the patch, where he decided to take a Pyramidion with a gat/flamer, and it REALLY was a bother, especially with balloon poppers his other teammates decided to take. You always either ended up with hull down from too much gat fire, or hull down from too much fire stacks, and unfortunately, you can't keep the hull maintained and protected effectively from both anymore, not with the current chem spray cooldown. It was frustrating to such point that I actually swapped places with the other engineer and took the fire ext as a sidegineer, assisting my main engie. And we won this one, I like to think thanks to this (not really, our ship still barely did anything, we just didn't die anymore :| )
And that's only one example, depending on all kinds of gun combinations, there is always a situation where, having a chem spray, you end up with a choice that either gets you killed, or gets you disabled with fire and killed later on.
I am kind of a person that is tolerant about all kinds of weird ideas for builds and layouts when it comes to competitive games. I believe that everything is useful, depending on the situation. I see the advantages of having all your engineers carrying chem spray, but my belief in everything having its use is the very reason I made this thread. I'm just wondering why noone takes into consideration that there are situations where you'd rather have that one fire extinguisher on your ship. I'm not saying in all kinds of situations, I mean what's the point of taking fire ext if all you're fighting against is hades/flak and such, but with flamer buff we have much more use of the extinguisher.