It's funny how one's own sitch can blind you to another's... so I never even questioned the moving out part 2TP. But it is something to consider.
In my own sitch I know when/if that comes I'll move out. It doesn't make sense to force three kids to leave their home and move just because mom and dad can't get along. And since the older two are stepchildren I can't very well tell my W to leave and I'll take care of the stepkids, since legally they're not my kids. I suppose I could tell W to take the stepkids and keep my S, but I don't want to tear him from his brother and sister.
So yes, there are exceptions and reasons why the LBS would be the one to move out. But all in all I definitely think about it hard.
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD