In my experience I use scouts all the time. Knowing the composition and formation of your opponent (In MP or SP) allows you to take advantage of it. Not only for combat, but also for monitoring movements of enemies and allies as they move around the map and take and build provinces.

PD is not very effective at all in stopping Scouts and even less effective in spies. There is also no reason to have a patrolling army in each and every province (as it's economically unfeasible). Also certain nations are naturally bad at patrolling (Ermor for example, or most Underwater nations forget or don't feel the need to patrol) so it's an advantage in that regard.

Scouts are one of the most important thought processes of my turns.