Hitech used the RV8 to compare to his real aircraft (which is an RV8). By coding the game engine, then seeing how it flies and behaves in comparison to the real thing, he's been able to get a fairly decent flight model running.
Most other game developers don't bother to do that.
The RV8 has been around a while, from what we were told. It was just hidden, a testing tool. So for that convention they stuck miniguns on it and released it for the convention-goers.
It is not a WW2 plane. Any time you see it enabled, it is a BUG, a problem with the default map setup.
It will never be enabled at any main arena unless HTC wants it to be. The odd fields you find it at are simply oversights in the checking of the maps.