You know this would be a neat idea. Sure would be easier on us all.
But, If they did it would likely be extremely expensive considering the amount of gold it would save us. Probably more than pat5. But in the meantime, I don't think they'll be implementing it anytime soon. Buying mercs seems to be intended to be a sinkhole for in-game gold. Amongst other things.
Either way, that's darn good thinking.