Very true but for the 8 bit Atari it took a tremendous load off the processor. It sounded like his intent was to program for the 800 or 5200. PMG was a hardware based sprite that really made the Atari much more powerful than the Apple II or Commodore in it's day.
I'm kind of curious why you want to use soft sprites, unless it's for ease of porting games. One of the reasons I got the A800 to start with was PMG, which was pretty much unique at the time. I don't think I could have written the one game I did get published with soft sprites. (Cosmic Defender in Analog)...