We have a futon in the living room
Well, theoretically, we have an extra bed, but only sometimes. I'm the snorer in the family, and my husband is a light sleeper, and he ended up on the couch so much we decided to maintain separate rooms for everyone's sanity
So if needed, and he doesn't have to work in the morning (and a good night's sleep isn't critical), we will offer the extra bed. My brother still lives at home and sometimes travels with my parents, so we could host them all if they ever took us up on it.
Ideally, I'd like to have our rooms and two extra - one dedicated to guests, with a pack-n-play in the closet (lots of friends with kids, including my pregnant best friend - I'd love to have a place for her to put the baby down so she has one less thing to worry about - and others, of course, but she's why I thought about it), and the other for the computers/books/craft storage. I'd put the futon we have now in the computer room, and buy a sleeper sofa for the living room. I doubt we'll ever have that many guests, but I'd like to be able to if the opportunity arose. I feel like an actual, real bed is critical, though - my mom has back trouble from her spinal surgery and I want my parents to be able to stay with us, and not need a hotel.
My parents didn't have an extra bedroom until I moved out. It's Mom's sewing room now and they put an air mattress down when we visit. DH's mom and stepdad have 4 bedrooms - master bedroom, SIL's/guest room (depending on whether she's living there at the moment or not), exercise room (it used to have a bed but I think they got rid of it), and MIL's 'woman cave'. His dad and stepmom have 5 bedrooms, one for each of them, 1 for the office, and 2 guest rooms, I think. I do know the office used to be the guest room, and the 2 guest rooms used to be for DH's brother and sister when custody arrangements were still in place.
Most of my friends (that have their own houses) have 3 bedrooms. My best friend's house used to have 3, but the previous owners expanded the back of the house, enlarging the master bedroom, adding a master bath, and enlarging the third bedroom and putting a back door in it that leads to the carport. So it's not really a good bedroom any more. Not for kids, at least! They use it as a computer room and as their main access to the house. They had a spare bedroom but that's being turned into the nursery.