The point is that @Mitten 's assertion that there's effectively just a big red button with "PUSH TO CAUSE PLANET REVOLT" is not true.If I put Enforcers at a low Priority, of course the Slave planet would rebel. Enforcers are Specialist class pops at leat.
Slaves might not cost you a lot of housing. The additional Enforcers you need to keep them from overthrowing you, do.
It is not ridiculously implausible that you might find yourself in a situation where you need raw resources for warfare stat, so you want to reassign pops from enforcement to mining, and 3 of your slave planets can handle that but 1 of them can't, so you grit your teeth and accept a high probability of one-planet slave revolt in order to prevent 5 planets being lost to the enemy war machine.
I mean, I agree that currently in 2.1 you have to be willfully negligent to get a slave revolt, but if the inter-mechanical balance is improved in 2.2 then maybe not.