The theory is that the QD spray encapsulates and lifts the dirt off of the surface to be wiped away by the MF which traps the dirt away from the surface. QD's (depending on which brand) contain emusifiers, surfactants and polymers/wax that do the work and also slicken the surface.
You must be able to discern how dirty the car really is before you use a QD. Light dust, road grime, water spots, fingerprints, etc. should be fine but once it really starts to get really dirty or you have driven in the rain, it's time for a wash. Stronger QD's such as Poorboy's Spray and Wipe and Chemical Guy's Speed Wipe start to blur the line between a QD and a waterless wash but I'd still hesitate to use those products on a really dirty car even though some claim success using them as a "waterless wash."
You must develop proper technique for QDing as well. Don't rub hard at the finish even with an MF. Let the product do most of the work and keep a fairly light touch.
The suggestion of a duster such as the California Car Duster before QDing is an excellent one though some here still do not like that idea and some do not believe in QDing at all and reserve QD for only after a wash to bring back gloss and slickness and/or aid in a mar free drying experience.
Lastly, stepping up one more level, you have the true rinsless wash products such as Optimum No Rinse Wash and Shine and Protect All's Quick and Easy Wash, both of which I have used on a dirty car with perfect results and some (myself included) use these as their main wash product but they could also be used in between washes for a quicker wash method when the car is too dirty for a QD and you don't want to or can't break out the hose.
