Well it takes a lot of flammability for anything to burn down. And unlike crime its a fixed number rather than
"per turn". And since its an event its still quite random to get a fire even if you reach the threshold. +5 Flammability is nothing.
Also every little bit of

,

,

and

help. Especially when build in multiple cities.
For instance building a Charcoal Burner helps in the long run if you build it first since production after that is less. Thus building

and

in a city will help them grow faster and thus produce faster.
Avoiding any Flammability, Crime, Pollution or Disease will hurt you in the long run because many of the buildings that have these properties have other benefits that boost you. That was one of Joe's problems. He avoided Crime so much that he could not get enough gold.
While if he had just built some anti-crime units/buildings then they could cancel out the crime and have a net gain in gold and still low crime.