The problem, as you identify it, is that Breath of Fire is just stupid AoE with no defensive upside, but is this a bad thing? I get that brewmasters require buff uptimes in order to be effectively tanking vs raid bosses, but many brewmaster abilities are irrelevant to raid bosses (Clash, I'm looking at you!) and that situation (it's a boss!) is the situation where trading survivability for damage is a bad idea anyways.
I suppose another way of looking at it is this: Do monks need another addition to their AoE rotation? It's clear from the recent buffs that using Keg Smash as the primary AoE tool is intended, and that it hits like a beast means that in full mitigation mode there really can't be a lot of room for more AoE that can mitigate damage, especially given that the only time you DO Breath of Fire is in conjunction with Keg Smash (well Drunken Haze, technically.) So if BoF gets a defensive boost, does that mean Keg Smash has to take an offensive dunk in order to make up for the additional use of Breath of Fire? And then if that's the case, does BoK or Jab or Tiger Palm have to be buffed in order to make up for the loss of damage from Keg Smash?
And if that's the case, won't that teeter WW and MW off balance?
Suggesting a buff to a class that can do something very well (Monks have NO problems with AoE threat whatsoever) is never a straightforward process, and 'because this ability does not help me do what it is not intended to help me do' isn't a good reason.
EDIT: A cup of coffee in my hand might help me understand what the real issue is then. It is said that monks NEVER want to trade survivability for damage. I don't disagree with that, and I think the issue there is because of the overreliance on active buffs compared to the other tanks. Give the monk a bit more baseline survivability, and suddenly you're looking at situations where adding dps is a viable option. It's not a simple discussion, and while I -do- agree the ability has little niche in endgame content, I can't write it off. It's very usable while questing, and perhaps that is its intention.