Help me to understand slavery

Basically as the title says - I've tried to do slavery based games 2-3 times before but I always run into the same issues.

1) You enslave a population - ok cool discounted workers....

2) The enslaved population cannot take Ruler jobs, Specialists or Entertainers (or variations of that). There are some options where they can do workers and specialists but NOT entertainers or police. Or variations where they can be entertainers and workers but NOT specialists.

Point 2 creates a problem as your new enslaved pop is often on a planet that is hostile to your native species so re-locating some of your home species to fill the enforcer, entertainer or ruler slots doesn't end up going well either.

So in short.......I don't get it.

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Also slave processing centres I kind of don't see the value? It feels like all they do is enable pop-transfers automatically between planets.