Has anyone ever used prehung fiberglass exterior doors on the inside? I always like to buy exterior rated knobs for my inside doors, because they last so much longer, so I was wondering if there was some reason not to use outside doors like this? I know I would probably have to remove the threshold that comes with them. If it matters, I will be installing a solid wood floor and using painted trim.

I have used insulated METAL prehung doors inside garages.  I ordered the doors without the threshold.  Maybe you can find fiberglass doors without the thresholds.

Thanks for the info. I hope I can order them without the thresholds.

I don't understand what you want to gain from Fiberglass doors on the interior. You can get a solid core interior door that will produce the same results with less cost. But you can get any door hung on interior jambs without the threshold. Just have to deal with a good door shop. Not a building supply warehouse like HD or Lowes. 

I was hoping to gain noise reduction and not have to repaint/maintain the door as often to keep it looking nice.

Noise reduction may be a valid reason to use an exterior door. Repainting? Wouldn't the same things that cause wear and tear of the paint on a regular interior door be the same for an exterior door used inside? Well, maybe they would resist teen boys' beatings better!

I can see the theory that you all are posting about using exterior doors for interior. But all can be achieved with the proper interior door. 1-3/4 interior solid core interior door is less than an exterior and will do everything that has been stated. Sound proof, better quality,  ability to use exterior lock and more resistant to damage. You can usually find this type of door for under $100, where a exterior will be over $100.