I've seen options like those at more than one occasion. Flats rented out by room or bed, with the owner? (operator?) seemingly running it as a business.

Don't get me wrong, I don't necessarily consider that wrong. After all it's likely a little more like a youth hostel and gives you the option to maybe meet with a pre-screened[1] set of occupants? Making an option to meet people with similar interests, whether that's something you'd want to go for or not.

For me Airbnb always seemed covered that middle ground between Couchsurfing and Ho(s)tels.

[1]: what ever screening the host would really do is a completely different question. But having to be approved by the host gives the host a little option to screen those who come and go.