I think the most practical application of the non flammable properties of natural fibers is the wearing of naturals as the layer next to the body when in a situation (like flying in small planes) where the possibility of dealing with a significant fire exists. I am told (and I don't know of any objective studies) that synthetics will melt on the skin and cause significant injury - injuries that do not result if natural fibers are next to the skin.

The biggest problem with synthetics around campfires is the damage to the very expensive garment, not to the wearer. I don't know of any instances of injury from a campfire involving synthetics as a significant factor. Does anyone else?

Overall, I think the most significant advantage of synthetics is their inability to absorb moisture. They can be wrung dry, worn immediately, with final drying resulting from body heat. Wool is good; it's just that synthetics are slightly better. Cotton is a different story.....