There is not proof of concept code to me. Every line of code that I write professionally I write it to the higher standards. That doesn't mean over engineering it, but it means writing it in a way that is maintainable and understandable by others. I can compromise on functionality, of course in a proof of concept I implement only the things that I need, but not on code quality.

Even if I have to write a script that I know it will have to be used once and never again I will write it in a good way, you never know if you need it again, if a coworker does need to do a similar thing and you can just give to him as reference, or if you can take pieces to do other things.