Over on WP Theme Tutorial I just wrote a post about why we document all our code for a project.
In short, you’re going to be a better developer because of it and you’re going to make it easier to work on in the future.
For our client projects it means that a future developer can pick up the project and have a big head start on what’s happening and the thought processes behind our work.
Skipping documentation is a good sign of a lazy developer. Then you have to wonder where else they cut corners.