That makes sense - I guess it's not as important for the wash water to be absolutely free of chemical impurities.

I have to admit I'm sorely tempted to get one of the water-filtration units like yours. On the one hand, I keep telling myself that it's overkill, but on the other hand, I look at my garage, and I already have rotary and orbital polishers, more pads than my wife has shoes, enough microfiber towels to go for weeks between washes, and 30+ feet of shelf space filled with various detailing chemicals, and gallon jugs of a dozen more, so who am I really kidding?