There's nothing worse than getting into the office, only to find out that one of the jobs threw an error and didn't complete. You get to a point where testing all the pieces in isolation just isn't good enough. I need to be confident that all of the pieces will work together.
We run on 6-week budgets for most major product work at Basecamp. It’s not that everything has to done in that time, but the self-imposed constraint makes us try. It’s not a deadline, mind you, but a budget.
Developing your "code nose" is something that happens early in your programming career, if it's going to happen at all. I combined all the documented code smells I could find into this reference; most of these smells should be familiar to you.