Agreed. The minute you know the feature does have traction and won't go away any time soon is the minute you clean or rewrite that code.

This may be semantics at some level, but I think it's important to not imply you just don't care about the quality of your code. Prototyping intentionally sacrifices some quality to prove a point quickly, but once the point is proven, you throw it away and rewrite with your normal, high standards.