Brandon Savage has posted some of his thoughts on when it's okay to write bad code in your development lifecycle:
I've been there myself. I recently needed to prototype something. As I sat down to work on it, I had absolutely no idea how I was going to write the component I was working on. And so, I started working - without a plan, without writing tests, without designing an architecture, and without really knowing how the component was going to end up. You know what? The component came out working, but when I was done it was ugly. Totally ugly. The code was bad. But I had a solution, and a solution that worked.
He points out that sometimes, doing things "the right way" can stifle creativity and experimentation - two things that a developer needs to solve the problems they face day to day. He notes that refactoring is a part of their job and moving from a rough prototype to a finished product often improves this skill and can find issues not discovered before.
This does NOT mean that developers can push bad code into a repository. Nothing lives longer than temporary code; see to it that your finished code is always good.