[ghaff]

I don't expect it's super common to have a dedicated guest bedroom in a house unless it's, as you say, a kid's room who has moved out. On the other hand, quite a few homeowners have a room they use as a part-time office, for hobbies, etc. that has a sofabed in it. That's more or less the case with me. I have a "spare" room that I use for various things but I can make up a bed if I have guests.