I dont usually use mercenaries, but there are a few exceptions:
-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.