I disagree with this so much, prototypes and proof of concepts teach you so much but usually they are crap you will always write it better a second time. Throw away the prototype and re-write it as a much better implementation.
People making these games are not in the games industry because they are not professionals (they don't live off these games). Also, it's curious how people who have not even shipped a prototype game down-vote opinions from the insiders. Good reminder when reading comments on something outside of my expertise here.
No. You won't:
I remember Carmack building a prototype of the texturing system for Rage to see if it would all work, so maybe the prototyping was always just an implicit part of JC's programming.
You can build things to try out and experiment different modes of play or modes of rendering but the code you write should always be production ready. No taking shortcuts because "it's just a prototype".