Naturally this would be a buff to the use of mercenaries, but since they spawn with no moral anyway it's not like you can just raise them on top of any enemy army and expect to win. And would also make it easier to deal with problems once you have a far-flung empire where it's hard to move troops quickly.
It would allow you to easily combine mercenaries with your army. I'd advocate the option to choose where the mercenaries appear at, within your realm, similar to how you click on a county to place a chancellor there. It's also unpredictable if you're capital is occupied as to which adjacent county they might spawn in. There's nothing more frustrating then when your capital is being seized, you spend hundreds on mercenaries, and then they're immediately crushed when an the enemy moves to attack them.