Slave soldiers are okay, weaker in then a gal in most stats(Voodoo being the notable exception). But they are low maintenance and easy to replace. I use them to guard lesser bases, and to fill roster gaps while the gals are in sick bay.
Early in the game, due to rank salary bonuses, staffing your bases with just gals can be cripplingly expensive. Particularly cause most of those gals will rarely see action.
Later in the game slaves do get some decent armor and are perfectly capable of handling lesser mission on there own. The recently added testudo armor actually has more sheer armor then Annihilator, not as good resistances thou.
If you play with the more tanks/cars mod then you can really get some mileage from the slaves as only they and loknarr's can "drive" those.