One of the things that draws me to functional programming* is the relative freedom from side-effects. A poorly designed, tightly coupled system begs to be rewritten, but doing so would come at an enormous cost. If a system is designed to snap together, with easily interchangeable parts, it's easier to be satisfied with evolving the software one piece at a time.

Then, when the urge to rewrite something strikes, you can just rewrite that one component, knowing that everything else should work just fine.

* I haven't actually built anything substantial in a functional language, other than my blog (in Clojure), so I can't back this up with experience. But I think it's true.