Then you're only doing half a job. Frames, sills, glass, they're all part of the window and all need cleaning. When they're not all cleaned it just looks crap; and you don't notice the nice, clean glass, just the filthy sill or frame.

And if you charge extra you're a crook as it's really only on the first visit that you have to actually clean the whites, the rest of the time they only require a quick wipe over with the brush.
I guess that why, after twenty years, I'm still very busy with existing and new work; quality speaks for itself