I never liked the explanation that it is "useless because it's multiplicative". It reduces the incidence of fire for every shell by 7%. If your original chance to be set on fire with a shell was 10%, it's only reduced by a flat 0.7%. That's still an overal incidence reduction of 7% though If it were a flat reduction, it could go a lot higher, yeah, but that doesn't necessarily make it useless. Yes, additive > multiplicative, but not all that is additive is useless. There needs to be an actual reason.

The uselessness of the skill is mainly in the fact that fires are spammable because ships spit out HE shells like tomorrow, and of such 7% reduced fire incidence is not very much.

Let's say a ship fires 10 shells. Let's say each of those has 10% chance of setting you on fire (including ship base fire chance).

Fire prevention reduces that to 9.3%

Without fire prevention, the chance that no shell would set you on fire = (1-0.1)¹⁰ = 0.9¹⁰ = 34%

With fire prevention, the chance that no shell from the salvo would set you on fire = (1-0.093)¹⁰ = 0.907¹⁰ = 38%

Hey, we now get only 4% less fire incidence per salvo!

But it gets worse. The main problem with fire prevention is that HE salvos get spammed. Let's say a ship fires 10 shells, at a rate of 12 RPM, and 25% of those shells hit a certain part.

In one minute, the chance that you don't get put on fire, is:

0.9^10x12x0.25 = 0.9³⁰ = 0.042

With fire prevention, that's 0.907³⁰ or 0.053

Now we only have 1.1% less fire incidence per minute, because so many shells hit you...

A compartiment that gets set on fire, cannot be set on fire a second time. Therefore, as soon as you get lit on fire, the fire chance reduces to 0 for that compartiment. Because of this, the 1.1% per minute doesn't really matter, because all that happens is you delay the unevitable by a tiny bit.