Finally I could take 4 slaves into combat. equipping each with the best weapon and armor, and training or rolling their HP to 100.
well equipped mercs are ridiculously expensive
This also helps with that issue, as you'll be able to equip your mercs with any weapon/armor! That, plus the fact that mercs can be trained (or train themselves, if they're not busy) means that if you're willing to put the work in, you can take a cheaper merc and train and equip them to equal a black guard, without the absurdly expensive upkeep! (I think. I dunno if Grim will change things, like having upkeep scale or anything.) Finally, Aiko and Enya won't be shit! Oh, and I forgot to mention that because mercs now work on the issid system, you can assign them the same duties. Like guarding, scavenging, etc.
Mind you, you'll still have to pay quite a lot up-front for the better armor and weapons, but you have to do that with slaves anyway.