If dirty code is well tested, it can be trusted. The problem is, that unit testing dirty code is usually very hard and cumbersome. This is why TDD is so good; it reveals and removes dirt and smells. Also, unit testing is often the first thing to suffer from time preasure. So if the cleanest guy ever made the cleanest code he'd ever done, I would still not trust it one bit, if he omitted the unit tests due to time preassure.