I find the war system quite tedious, especially against larger nations with a bottomless well of manpower - a nice shot of merc spam generally gets me by. Nothing more irritating than looking at the 'are you sure you want a war screen', accounting for 200k soldiers, and then finding that the AI dumps all of their professionalism and wrecks their economy to double / triple their force size within a year. No wonder the late game stagnates - the AI treats every conflict like Operation Barbarossa.
Mercenaries are also useful for the endless 'hunt the overequipped rebels' jobs. You might have zero manpower in reserve, and be running a deficit as you beat on Austria-Hungary for the tenth time in some boring HRE fight, but those Catholic Zealots in your heartland will absolutely find 20,000 good infantry, 6,000 cavalary, 8,000 cannon, and a two-star leader to screw around with.