Depends on your point of view. (Score 1)

From a Venture Capitalist PoV - Never! Unless you gain something out of it (A competitive advantage, reduced cost, etc.) From a Dev PoV - If everytime you need to make a change you dread it because it means many sleepless night debugging some archaic code, you need to refactor the whole thing (Which may involve complete re-write if everything is so badly designed) From a QA PoV - Right on! More stuff to test, re-write every week! From a Management PoV - Somewhere between those lines, you will find your answer.