In short: You have a happy, reliable workforce / think-tank available, with minimal risk of rebellion or general disorder. Some of 'the weak' may become strong and may become an asset to keeping your government going. In fact, if you encourage everyone to do the best they can, you will not have _weak_ people.

Slavery is a flawed system (other than for the moral reasons) because it creates an 'half-assed' work force that at best will do an half-assed job because they're not motivated to perform better. They do what they're told, nothing more. Plus, if you really push them, live expectancy is low forcing you to rebuild at regular (economically unhealthy) intervals.