LowNSlow,
you still have these old synthetic fabrics in mind, there where few cases where they were burning when exposed to flames. Most synthetics don't have these properties anymore, otherwise they would never pass the rigorous testing - well, if they come from China I don't know
Newer polymers rather melt than burning. And it has to be very close to the flame. If you are so close to an open fire, you have much other problems... - try to grab a Dräger Oxycrew for example!!
I also disagree that cotton "breathes" better in hot situation. Try to wear a Nike or Asics T-shirt on a hot sommer day and exchange it with a cotton one, you feel the difference. If these synthetics are worse, why would all marathon runner wear them? And not only them.
Dani