My last coach, a Vectra, had no bottom part. The blind was secured to the top of the box and the wall at the bottom (with eMike's doo-dad). Most coaches are this way.
The higher end coaches tend to have the bottom piece. It helps to cover the bottom of the window's aluminum frame. It looks a lot better, too. Yes, it is a dust trap, but the "look" is entirely different.
Now when I go into a high end coach and see the bottom of window frames, my mind says, "why did they go cheap on that?" I suppose it's all a matter of taste. I still prefer mine, as is, dust and all.