I like it. So long as there's a hard limit of the number of guards you can buy, so we don't see the utterly insane amount of slaves some people ended up having on their bases.
In fact, it'd be nice if mercs were equally permanent investments. If they die they maybe disappear for 24 realtime hours, but then you find them again in a random encounter. It would be a nice improvement for charisma characters, and we'd see a lot more people buying them.