The requirements for building these would be lower, and they would end up renting somewhat more cheaply. I know a lot of people who don't own cars and walk / bike / taxi / take public transit everywhere, who I think would be happy to rent units classified this way.

Almost everything else about cities can scale with more money, and when you build new units you get more in property taxes. The main thing that doesn't scale is space, and by allowing more people without further dividing the space we should be able to make everyone happy?

[1] For example, my alderman's most recent monthly letter contains:

"21 Cherry Street — 6 units. The developer wants to bulk up the building and add many bedrooms without additional parking. The neighbors are opposed and have urged the developer to scale down the plans. ...

39 Murdock Street — 3 units. The developer is expanding a small single family with less than 1,000 square feet into a large 3-family with 10 bedrooms and almost 6,000 square feet. Neighbors are concerned about parking issues."