I'm still conflicted about this. On the one hand I agree. On the other, I wish we could educate people enough for them to understand Proof of Concepts and Minimum Viable Products.
You must demo the product in a form that leaves the right impression of the current state. If you are painting a picture of a polished product, expect polished expectations. Instead, show the bugs and say "we are still working through this section" Show missing pages, show your work in progress. Show wrong colors. Show potential, wave your hands and tell them to imagine this part working. Don't fake it. If that feels wrong, then a demo isn't right.
If you show it in a form that looks complete and polished, even if you say it is not, how can you expect any other conclusion from the viewer other than "it's practically ready!"?