So Pete is your brush dirty or clean when you lift it off the glass? Personally mines is clean and don't need to lift off to rinse separately, so no problem for people who use it that way. If it's not clean and you see a reason to lift off, then resting it on the frame below the window is not going to cause any problems, and your brush is not going to end up any worse off for it. If your brush is clean and you still lift it off to rinse separately then its certainly not going to be clean as soon as it hits the next window that it is needing cleaned, so again no problem there with it touching a frame below a piece of glass that's just been cleaned. Most people clean the frames anyway, and those that don't probably don't because the constant flow of water on the frame below the glass keeps it relatively clean.
We all work in a certain way, with different equipment, and we all have different techniques moving around the glass, cleaning rinsing etc. We all have the odd awkward window that we have to go about in a different way. What I am offering people is an alternative to what they do just now, a very simple way to control the water, on and off, and reduced flow. the only way that you don't have to stop what your doing to turn the water off going from window to window, with your hands on the pole and your eyes on the glass.
Now there may be a time when you have to adjust your technique to make something work the way it needs to. Some might have a problem with that and not willing to change, some will see the benefits and change willingly.