So they want to sleep at your place but are not comfortable having you around?
And if they have back pain they can't sleep on just any mattress either, as it might be the wrong hardness for them. And you might have another friend staying over so now you need two guest bedrooms for them both to have privacy, and both of which need fancy specific mattresses, ah and what if it's one of those couples that don't sleep in the same bed so...
Having an entirely unused room to cater to a very specific situation in which someone has to sleep in your house but also wants a hotel experience with a particular choice of mattress is a waste of effort. Guest bedroom by themselves is quite an American luxury thing. They're not at all a thing in Scandinavia unless you live in a mansion, and even then it would be weird waste of rooms to make spare bedrooms. Friends sleep on couches or fold-out beds if they need to sleep over.
> So they want to sleep at your place but are not comfortable having you around?
Yes, this is extremely common. All of my in-laws would go in this category, more or less. I might spend the night at their house, but it would be awkward to wake up with them in the room. Many friends who have moved far away and I don't see them regularly any more would also qualify.
> And if they have back pain they can't sleep on just any mattress either
There is an enormous population that exists between "can't sleep comfortably on a couch" and "needs some specific kind of mattress."
> an entirely unused room
Space isn't at a premium where I live, a whole unused room is no big deal. Plus if I have a another child there is a room ready for them.
That's the source of a lot of guest rooms, they are rooms for future/past children. My own guest room is also an office when there aren't guests.
If a space is repurposed from a previous permanent use, sure. Although, nothing is free, it's a more reasonable situation to end up in.
But then we've also deviated from the discussion of acquiring things with the intend to perfectly cover an edge-cases. On-topic, that would be like getting a long-range car because you drive the 1000 km constantly rather than for trips, and later lose the need and only use the extra range for leisure.
> There is an enormous population that exists between "can't sleep comfortably on a couch" and "needs some specific kind of mattress."
I'm actually not sure there are many people in that group. I don't think there's far between a decent couch and an "eh" mattress, closing the gap of people that cannot sleep on one but can sleep on the other. A decent futon or fold-out bed also goes a long way, and to theorize a bit I expect those that would not be able to have acceptable sleep on such would not sleep well outside their own home and bed regardless.
Either way, futons or fold-out beds are quite reasonable propositions comfort-wise and I sure wouldn't consider providing more for friends and family that needs to sporadically stay over at my home.
Home office with a fold-out bed/couch or futon, yeah. Guest room with a real bed, no.
I have the former, but the person clearly indicated that a couch would be insufficient to address back pains, in which case it sounds like they expect a copy of the master bedroom.
This works until your friends start getting married, start having back problems, or just want a little privacy while they sleep.