The phone wouldn't be but my tablet that I connect when I need a quick internet connection and a better more full featured experience would be.

I almost never use any of my smart phone's features except tethering.

The phone's screen is too small for comfort I would rather use my nexus 7 which I always have with me anyway however a phone of that size would be too awkward so why not just separate them.

A dumb phone with modem capabilities and great battery life and a smart device for everything else.