I use mercenary armies when I need (or feel like) opening a new theatre of operations, am far from my troop producing centres, and happen to have access to enemy generals bribable by my diplomatic shock troops. E.g. I raised an entire full stack from 3 ex-germanic factioners, 4 barb cav mercs, 2 Sarmatians and barb/thracian/bastarnae mercs for the rest to fight Germania, as Macedon. apart from getting drastically different tactical possibilities from my phalanx style, it also enables you to conquer the world faster, especially in a theatre that does not have a major importance, and does not require speed.

Con: not retrainable. But who cares? They aren't made to last, and with that army you can already last through perhaps 3 or 4 settlements, by which time you should have begun replacing them with your buildable units. Expensive, but instant. Useful for emergency defences, or take-at-all-costs-before-the-enemy-does.