No good reason? Reducing the bandwith used in many common situations (when you don't run a game or a video) is a *very good* reason.
That said, you do have a point that at some point the bandwith used by the rendering commands(and textures) could be higher than sending a bitmap.
So the "ideal" (but complicated solution) would be to send the rendering commands&texture when their size is smaller that the size of a screen bitmap otherwise send the bitmap.
Of course, this is made more complicated that you can send the diff between two bitmaps or/and compress the bitmap, but this has a latency cost..