How common is it to have a demo at a doctoral thesis defence session? It is customary (at least from where I'm from) to have undergraduate and masters thesis viva voce with project demos. So far, I have never been to a PhD thesis defence viva voce which had a demo in it. 

How (un)common is it to have a demo at a doctoral thesis defence concerning the fields of engineering, technology and especially computer science? 

Edit:

A project demo under this context refers to the demonstration of an important contribution put forward by the thesis defender. It may include one or more of the following 


Practical application of concepts
Simulation of proposed algorithms
Live usecase demonstration
I've seen a couple of PhD defenses with demos (in engineering/computer science). 

It's not common because many PhD theses don't naturally require a demo due to their subject, or because a demo would be logistically difficult (e.g. requires special equipment that can't be brought to the room where the defense will take place, or takes too long given the time allotted for the defense). Also, live demonstrations are somewhat prone to failure, and a PhD defense is not a good time for a failed demo :)

But if a major contribution of the thesis happens to be something demonstrable, and it's not too difficult logistically, then a demo can definitely enhance a defense. (If logistics are the issue, a short video of the demo can also enhance a defense.)