Pedaling constantly gives you a better workout, developing muscles better. Pedal-then-coast keeps you from getting in as good a workout in in the same amount of time...whether you get to your destination faster or not.

Most people want a good workout when they ride.

Ergo, keep pedaling. It is one thing if you're on a hilly century and spent and letting gravity do your work on the downhills, quite another if you're on a 20 miler and coasting on flats. Best way to see exactly how guilty you are of not getting good workouts in, look on Garmin Connect at the graph of cadence....if the graph looks like a rack-gear on your car, it means your endurance is getting short-changed. Odds are you're short changing yourself by coasting a TON more than you think you are.