In my experience, this is a trap.

The demo almost always becomes the product, because upper management sees the demo, and says "Hey! It's almost done! Let's ship!"

This is how you end up with these gigantic Frankencodebases.

These days, my test harnesses and demos are generally "ship quality." It also means that I can mine them for snippets, without holding my nose.

I have spent a great deal of time, however, practicing, so that I write top-shelf code, by habit. These test harnesses are often churned out very quickly.