Rewriting has a concave payoff. It requires high levels of certainty to be valuable. The bigger the rewrite, the more likely you're less certain than you think.

Rewriting has a concave payoff. It requires high levels of certainty to be valuable. The bigger the rewrite, the more likely you're less certain than you think. Refactoring is the same as rewriting, just on a smaller scale, which increases certainty. Joel's advice assumes you will overestimate your certainty.