OSX does defrag on the fly, but only for smaller files (sub 20MB or 25MB, I forget which). To be honest, this is fine for most users but people with smaller hard drives working with larger files might benefit from a third party defrag app. However, since storage is so cheap, this shouldn't be an issue for most people.
Repairing permissions is something to do if you're bored or if the machine is acting slightly quirky. In many cases it works, in many cases it has no effect. It's easy to do though, so should be considered a good first step.
Periodical scripts are also of little importance to most users. Once again, I wouldn't bother running them unless something was acting quirky. Of note, these periodicals run automatically, very early in the morning. If the computer is shut down, they will not run. If the machine is asleep and running Tiger, they'll run next time the machine is awoken. 🙂