I tend to fall into the theory that by letting the car sit with ionized water on it while it dries, pollen, and other airborne particulates settle onto the car that would normally get wiped off by towel drying (and swirling the paint, but thats another matter).
I've used soap other than Mr Clean's in it and that worked just as well.
You wet, you soap, you rinse with regular water, then you set it to "ionized, filtered", and rinse again. My bet is the advertised lack of water spotting comes from zero contaminants in the final rinse water.
There seems to be no sheeting action additive, or any additive at all to the final filtered water.
Still researching, but I'm finding nothing negative.