Chicago is chock-full of old houses converted to three-flats. Most of those flats have large balconies on the back... with a staircase. These balconies provide fresh air and nice views at a price point where both of those things are otherwise impossible to find.
So yes, my counterpoint is "but I like the second staircase." But, I really do - a lot of people do, and I don't see why landlords would build those things if they weren't required to.