Dirty. Nasty. Interesting...
But to what end???
Drones are cheaper to use as an army.
Wimps are good enough for medical testing.
Sinless are cheaper for manual labour.
Not to mention you don't have to worry about a spirit getting bored, or growing a conscience, or worse... And deciding to take your regenerating body and ram a leg up where the sun don't shine! (don't forget, spirits aren't mindless automatons, they have their own ideals, values, morals, wants, desires... And at higher forces, are perfectly capable of out thinking/smarting a summoner and his goals, if they choose to do so...)