Humans always make mistakes and are tired now and then, and the smartest people can still appreciate not needing to solve puzzles just to understand or debug their own code.

Of course clean and readable means very different things to different people, but I don't thing I ever regretted cleaning up code, while I can think of a lot of instances where not doing it wasted a lot of time.

If other people need to read your email/doc/code, then any effort to make it more readable will save them time, regardless of how smart they are.

"If I had more time, I would have written a shorter letter."

Sadly, I think this is true of me — at least of me. I don't think smart people (by this definition) are that common though, so clean code is a sensible default.