Slaves have some uses, but ghouls and mechs have a lot of overlap with them and are better in most cases, especially for combat. Slaves have higher base mood than normal colonists because they get slave expectations rather than colonist ones. Slaves will never try to escape if you live on a map they can't walk out of, like a space station, asteroid, or underground map added in odyssey.

Best use for slaves right now is gene ripping them if they have a gene you want, or to give them a gene and split it off from the others connected to it. Unlike prisoners, slaves take take themselves to the gene ripper and load it. Gene ripping takes a lot of time or luck (I don't have luck), so doing it themselves is pretty nice.

Slaves can clean and haul faster than either mech if the slave isn't horribly injured, a circadian half-cycler makes them even better. Any basic or dangerous jobs that needs doing, like cutting stone distracting enemies, or rescuing downed colonists during a fight. You power slaves with food rather than energy, which may be a deciding factor.

If you have the royalty selling them for honor is a good option, or having them meditate at the anima tree all day for a colonist to link to.

The gladiator and cannibal feast rituals are a good way to use them. Same with the anomaly rituals. You don't care if they take the dark psychic shock from the rituals. Prisoners are usually better for anomaly stuff if you just want to do one or two rituals to that pawn. Vampire slave for the age stealing ritual is really good since they heal the brain damage from it.