Mercenaries and what you can do with them

[Prussian1]

-If I attack a province that has at least a castle in it, I usually hire some siege-equipment mercenaries to use if i have to assault the castle since I dont normally bother to build those pre-gunpowder siege-equipment myself.
-If I see some special units (eg. italian inf. on early or some nice shocktroops like gallowglasses) I might hire them if I can use them almost immediately.
I disband my mercenaries right after they have fulfilled the task I needed them for, because the support costs are so high.

Well thought out. I use them for very specific blitzkrieg missions and let them go a few at a time as I get troops trained up in the newly taken provinces. With the exception of units I will never have access to, I try not to keep a mercenary unit more than ten turns.

[Noir]

Mercenaries give an extraordinary flexibility to the human player as they are. They give the advantage to quickly transform earnings into military strength and be able to disband them whenever pleases the player, both things impossible for the AI. They are an interesting game element nonetheless and offer potential for modding.

Noir

[Martok]

Like Haccapelite, I will hire mercenary artillery crews from time to time, particularly if they're trebuchets or mangonels. I also admit to adding mercs to Crusades, although I usually limit myself to special units such as Vikings, Welsh Bandits, etc. Aside from that, however, I avoid using them.

Mercenaries give an extraordinary flexibility to the human player as they are. They give the advantage to quickly transform earnings into military strength and be able to disband them whenever pleases the player, both things impossible for the AI.

Which is why I limit my use of them. Mercs are a fun gameplay element, but they put the AI at too much of a disadvantage.