This is not necessarily true. Some people rent out the living room couch as well as the spare bedroom. Besides in the second lessee case, the landlord at least gets to vet the second person.

Vetting only lessens the chance of catastrophic damage from an idiot tenant, though. It does nothing to mitigate wear and tear. That was what my now-masisvely-downvoted point was. The "wear and tear" argument is on pretty shaky ground.

The wear and tear argument was not made along with a vetting argument. Vetting is one issue. Wear and tear is another issue. There is some ground between the perfect tenant and the idiot who burns a place down. There are all those people in the middle that go about their lives and little things happen. But have you ever heard the saying "drive it like its a rental"? People will naturally be slightly more careful with things they own or have a direct responsibility to take care of. The more people you have using a space that are not directly responsible for its care, the more likely those little things will happen. We're not talking about the catastrophic damage. We're talking about things like the extra nicks in the banister caused by someone dragging their suitcase up the stairs. When you have a new person dragging their suitcase up (and then back down) the stairs once a week, the chances for additional nicks goes up. Eventually all those little things start to build up. And it is pretty easy for a one of those rotating guests that caused a little more damage to just move along since its not their apartment building. People who live there and have a responsibility for the place (and coincidentally have been vetted) are more likely to notify the landlord to get it corrected.