My observations:
With wfp the windows do stay cleaner for a longer period between cleans.
Just like with dry cleaning a garment or a car or anything. There are different methods of cleaning that will leave a different finish.
To illustrate: I can wash the car with wash 'n wax, I may decide to do that every 2 weeks, but the car probably needs doing again after a week. Or I can wash it with purified water, it will stay cleaner for a bit longer, but will still need cleaning again in 2 weeks.
With the illustration of the car, it will always need the clean at 2 weeks (or whatever is your preference), because it travels on dirty roads at speed.
With windows, I've noticed, if theres been hardly any wind or rain, and the birds are constipated, they are cleaner when I next call, but those times are rare. Not often enough to go to longer intervals.
If customers want to go longer thats ok by me, though, there usually the type I don't look forward to seeing anyway.