what clean the windows then clean their cars?
i clean my van with a brodex brush and a converted small trad pole.i dont use the same brush for windows (as i scrub the wheels and i could contaminate the brush with oil etc).
i wouldnt do it although if your a scout doing "bob a job" it would be perfectly acceptable!!

i concentrate on mainly cleaning windows coupled with fascia/soffit/conservatory roof cleaning.
some guys finish doing the windows and get their lawnmowers out!ive had a fair few "odd job" leaflets through the doors lately offering allsorts from dog walking/gardening/tree felling etc etc.
i prefer to concentrate on the core business of window cleaning.better doing one service really well than jack of all trades IMO.