Rewriting the code from scratch does not necessarily mean clean-room reimplementation! It doesn't even mean you can't include parts of the original in the new code.

Rewriting from scratch means rethinking the design, or basic structure, of the code while keeping all its functionality. It's not about getting rid of hacks, it's about lining them up neatly in one place instead of having them all mixed, twisted, nested and threaded through the code everywhere.

Well, at least if you know what you're doing, instead of just randomly deciding on a rewrite because it's more interesting that way or because you can't be bothered to read and understand the original. Then, I agree completely with Joel, it's most of the time a grave mistake which made countless companies and developers fail.