The Negatives of Enslaving an Enemy People

The Greek city of Sparta conquered Messenia in 650 BCE. The Spartans found out that enslaving an entire people instead of massacring them has both positives and negatives. On the upside, you free up a lot of spare time to design social system where everyone spends all their time doing military drills or choir practice. But for me, if I all of a sudden had a lot of wealth in spare time, a regimented military life would not be my first choice either.