Bob:
I think you hit the proverbial nail on the head. Any sander that has "through-the-pad" dust collection is going to work best at collecting dust if the pad is flat on the surface being abraded. If you are using a flat pad sander and have part of the pad off the work surface, increase the suction to compensate for the free-flow of those suction holes that are exposed. You should be able to tell when you've increased the suction too much (if that's even possible).
The RAS uses a completely different suction policy and that's what makes it special (aside from pure rotation) for tasks like edge work, sculpting, scribing, etc. The dust extraction on the RAS doesn't depend on the pad being entirely in contact with the work. It just requires that the partial-round brush be positioned properly (in the 360 degree rotation) so that it catches what is flying off the abrasive surface.
Depending on how you have the rotating abrasive disk contacting the material, the stuff removed will go a different direction. All you have to do is rotate the brush to be on that side. Rotation of the brush around the sanding head is accomplished by rotating the side-grip handle.
Tom