This has been asked for before (so it isn't exactly something new), but it has been awhile and it's something I'd absolutely love to see added.
Deciding which Sims are to be non-playable could be set before you added your Sim(s) to the household OR you could simply set this once you're in-game. If you choose to set this while moving your Sim(s) into the selected house, there would be additional options to either keep the current members as playable or non-playable.
I remember looking over the houses in Sunset Valley that had a load of Sims living there (cannot recall the name of the households, but I'm sure you've all seen them) anyway... I was about to add my Sim to one of the households thinking how cool it'll be to have roommates, then it dawned on me quickly that I'd have to play ALL of the Sims in that household. It kinda put a dampner on things.
So, yes, it would be fantastic if we could do this.