I think you are missing a few use cases where single hand is needed, or at least can be used by some people.
Phone is one of those obviously, but there are a lot more. Basically everything done on the go while walking can fit in the single hand usage. There are a lot of time when you need using your phone while having a bag or a child in your arm. Checking GPS for an address, quick sms or mail answer on the virtual keyboard, taking a photo of something you handle, changing bluetooth settings for your earpiece...
And it can be quite hard on bigger screens (for some people at least) to reach comfortably the flash or exposure settings of the camera app, to active the gps in the events' pulley menu, or simply to pick the contact at the top of the list to make a call. That is something where the UI should help, but even Sailfish is not enough on some points.


PS: this post is only to add details to the screen size subject, not to expect Chen to change the specs which must be set by the time.