Or you'll get blamed for having to take a long time to perform the smallest change. I remember once dealing with such a system.. took me a week to change the price of something. People got all excited when I finally got it. Scary stuff.
One time, I did get permission to rewrite a piece of that system... you were right, people did get annoyed when new bugs came up. BUT, after the bugs settled, it was the fastest and easier piece of the puzzle to modify. Some time passed, and they were doing efficiency analysis on the system to see where they could improve on it. Turns out the part that got rewritten was skipped since it was the fastest part of the entire thing. :)
Yes, tooting my own horn, but thought I'd share the positive experience of rewriting, and the negative of not rewriting. There /are/ times when you shouldn't. Just not never.
Bart: God, Schmod. I want my monkey-man.