The only time I've seen it happen is with the butler....he/she is non-controllable but lives in the house (provided you assign them a bed).
It would make sense to be able to make others as non-controllable for those who don't want to be bothered with micromanaging every single person and just want to control 1 Sim.
Cranking up the Free Will works to a point, but I still find myself checking them to see what they are doing and if they are doing something stupid or going against character making them stop. My Sim's son is a loner, but whenever I leave him on his own, he starts chatting with others and gathering a group around him...which of course makes him miserable.
If you're gonna be a bear, be a grizzly