With demon shine you do still need to wash the car first and let it dry. Every now and again the car is filthy and I give it a good wash outside, then clean the inside and by the time the outside is dry I can't be bothered to wax it, so I resort to demon shine (if I can be bothered to do THAT!) and just get a bit of a shine.
EDIT: Just to add I watched mighty car mods episode and they mentioned a 'spray detailer' which I guess is what demon shine is. They say it's great to use between waxes. So yeh, I guess if you wash your car every week you might not want to wax it every damn week...you get the drift