I'm not sure I see what the difference is between having them as NPC's vs. having sims with full autonomy on that you generally ignore would be, unless you're thinking it would free up some slots in the household? Or to be able to play in lots you have set up as row houses or townhouses or something like that, where you'd have 3-4 different families living in the same lot.
Because right I do that right now, more or less. When I'm focusing on a particular sim's story, I mostly rely on autonomy for the sim to take care of their needs and only check in when my focal sim is doing something that takes a while or sleeping. They may as well be NPC's.
But then, I seem to be luckier than most when it comes to autonomy.
