It's a famous Joel Spolsky essay: Things You Should Never Do, Part 1 [1]. Never do a full system rewrite. Pain and sadness follow.
[1]: http://www.joelonsoftware.com/articles/fog0000000069.html
[1]: http://www.joelonsoftware.com/articles/fog0000000069.html
I think that the real lesson to be learned is that you should beware of rules that start with "always" and "never".