it′s more a limitation of the engine I guess, maybe it simply can′t handle changing textures, is this case I would say it′s more likely that if they improve how NPC handle fire, at least I hope they actually make them run around in terror.
It IS quite clearly possible, since mods have been able to do it, in fact, even the Oblivion version of the engine could allow this to happen with the proper scripting, so I don't see why Obsidian couldn't, especially since they can likely change aspects of the game that modders can't, since they're developing it.
I would like to see this, as it always seemed kind of silly to me that people who were burned to death by a flamethrower looked like they could ave just been poisoned for all the visible damage they show, while this problem certainly isn't limited to Fallout 3, it feels more out of place here than it did in Oblivion since in Fallout 3, the bodies of enemies can be dismembered, it just feels weird when people killed by a gun or melee weapon could lose their limbs while people killed by fire don't show a single burn on their bodies. Also, I think I recall that in Fallout 3, fire would cause enemies to bleed, they should really change that in New Vegas. Unfortunately, I haven't seen any sign to indicate that this has happened, though if it's not in New Vegas maybe it can be in a later game.
still I doubt they left out burning skins becuase they can, if it′s not in the game then there is a reason, anyway once the game been out a while the above moder will most likly make a NV version.
If they're going to start not adding things on the basis that modders will add them, then they might as well just release the "game" with the gameplay mechanics and engine finished, but just an empty wasteland for the world and leave it up to modders to create content for it.