They are not. Polygons are a terrible representation since unlike triangles they do not cleanly represent a unique planar surface. With more than 3 points you will always have an ambiguity (or several) about which (numerical) plane corresponds to the actual face. For some graphics applications this may or may not matter much, but it is very important for anything using the mesh for physical computation.
Co-planar quads will always subdivide into two coplanar tris. That's the crux on why the modeling work flow works. The GPU is going to turn it into triangles anyway as long as a few fundamental rules with indices are up kept, so you're mostly getting the best of both worlds here.
