In my opinion at whatever height you are working you know that with a good flow rate that the last thing to touch the glass because of where the water is fed from - will be 100% pure water.
I still cannot see how a brush that may hold impurity in its bristles can do as good as our assembly because when we rinse we never pull the brush over the glass we are rinsing.
Rob 
Neither could I if indeed the brush did hold impurities in the bristles, but it dosn't. Everything is being constantly washed off the glass AND the bristles as you work.
Apart from the disadvantages on overhead sprays that I have already mentioned, unless you are doing a downward stroke all the way down from the top of the glass to the bottom, then you are still moving dirt about with your brush. Watch what your brush is doing the next time it's on the glass, and you will find that a spray above the brush is, as you put it still holding impurities in it's bristles, so will be spreading the impurities to parts of the glass the the overhead spray has already been.
I think you will find that the overhead spray is working much the same way as the fan through the brush, as far as the cleaning, or keeping the brush clean is concerned. You are still going to get problems if the brush dosn't allow the water and contaminants to pass through the bristles and make it's way to the bottom of the glass.
The fact being and the point I am trying to make is, the bristles are being kept clean during normal use, just the same as the glass is, and whether the water is coming from the overhead spray or inside the brush makes no difference. And if you use an overhead spray instead of a fan through the brush you are going to have to deal with extra problems. Well that's the way I see it anyway.
Peter