x_x; Have the people saying it would add fun have actually tried it? Oxygen is combustible. Real-life fires suck in the oxygen, thus perpetuating the flame.
Fire in TPT generates atmospheric pressure.
Which means fires will push away their fuel. Can't you see any problems here? :<
*edit: Ehh well it wouldn't be absolute, but still this isn't quite as simple as it looks. If you light a fire in a bottle with an egg on top, the egg gets sucked inside. Requiring oxygen particles in order for flame to exist is a lot harder than say, not permitting fires to exist in < -200 pressure areas.
I can see how this would be useful, but the coding difficulty, save breakage, and all of the users that don't think it is a good idea outweigh the usefulness.