It's fairly rugged in standing up to forces applied in certain ways and from certain directions. When even a slight force is applied in another direction, the screen will break very easily.
If you twist the device (easy to happen if put haphazardly in a bag), or something sticks into the screen and applies some force, the glass substrate part of the screen will crack, and the screen will be kaput.
Basically, the two protective qualities you want from a case are to give the K3 some rigidity, and to stop pointy objects getting at the screen. Wedging it between two books in your bag would serve the same purpose, as long as you could be sure it wouldn't become unwedged.