First of all, I like this mechanics. It’s interesting and immersive… but not rewarding past first levels of your char. In fact, why should I need Exile fighters (even T4 Fighters) when I already have a decent T3-T4 Pirate or Black Corsair one? Now there is only 1 spot (as far as I know) which is worth checking - the one where you can get a named Bearer from the cage, the others are totally useless.
I see two ways for changing this (and one more that looks impossible). Impossible one is to get crafters from those cages. Unfortunately it would be too complicated for devs. But there are two more ways, not so complicated.
First is to give “rescued” Thralls at random some really good weapons and pieces of armor, maybe even legendaries. So when you set them free you’ll get your reward.
Or even better: make different cages for different factions. Exile fighters should be in prison in Exile camps. When you get to Black Hand camps, ok, there are some Exiles caught by the pirates - and some Drunk fighters who were put into cages for drunken brawl. Stygians should have Exiles and Black Hand caught in the wild - and some of their fellows put in prison for disobedience. The same for Accused faction as well. So when you visit a high-level camp you have a chance to get a really good fighter there.
By the way are there any cages where you get dancers? It would be useful as well, many of us like to decorate their bases with dancers.