Mercenaries are almost strictly early game. Even mid-game the stacks, besides being hilariously overpriced, become way too large. You would never need 66 infantry and 12 cavalry in one stack, ever.
The free company is nice though, it's especially great for some of the weaker starts where you have a few crucial wars in the early game where you need to go over force limit. (Kandy, Oman, Mzab and Wallachia come to mind)
The other companies are kind of eh. Early game when you need them they're too expensive, late game when you can afford them you don't need them. I guess they have a niche for wars between mid to large sized powers in the early-mid game, but then again if you're playing a major you should focus on the small fry before you fight any wars where you'd need mercenaries.