A question popped up at RFO the other day. A member found Revit offering him a family as a possible workplane to chose when using the Set workplane option. It's perfect for the Dept. of Subtle!
When you use a family as a host for a Face-Based family this new host status of that family causes it to be elevated in status, to being eligible as a workplane for selection now too. For example I've placed a desk and then decided to mount an electrical outlet on the inside face so I can plug in my little space heater (I'm making this up as I go...).
You can see the Desk is now offered to me in the Workplane dialog. So don't panic if you start seeing families in this dialog. It just means they are hosting a party.