The Myth of the Unfragmented Disks
Read any post you like. Read any _place_ you like. In 1994 it was "NTFS doesn't fragment by design, it never fragments". In the later 90's it was "ext3" or some other filesystem "doesn't fragment". Later, starting in 2000 - OS X doesn't fragment.
It started for me Windows in about 1994, and hit a sort of minor crescendo around 2002. I was speaking - ON THE PHONE - with Microsoft Technical Support. They were telling me about how fragmentation doesn't exist on NTFS, and that the new object linking would allow persistent data across an entire network. Whatever that means. NTFS not only fragments, but it HYPER fragments. There are some excellent write ups on this phenomena, but it underlines a few points. The first is that designing fragmentation proof systems is a process of revision, at least in part, and not necessarily all "design".
Pearls of Wisdom dispensed at the tail ends of sanity constantly admonish n00bs that Mac or Linux won't fragment, even if of other platforms will. Old school boards and such used to offer identical advice to the WindowsNT crowd. The sad facts are in. EVERYBODY FRAGMENTS. It's a function of longevity, slowly growing files, LARGE files in general and use patterns with the disk. Filesystems fragment very rapidly when you start running out of space, and no matter how large the volume, space eventually gets exhausted.
Leopard does well, but even it has limits. My 700mb Thuderbird inbox has well over 4000 file fragments. (I run out of space a lot). I have several .DS_Store files which have more than 3 fragments. If you have space, if it can be done fast and if it's under 20mb, Leopard will defrag most any file on the fly. That said, if you get low on space, make your computer extra busy, and then handle large files, you're doomed.
The unspoken dilemma
One of the things which Windows Defragmenters have taught us is the value of reorganizing the disk layout. I can't tell you if it's honestly that valuable, but I can tell you if gives a warm fuzzy feeling.
There may be many ways and reasons which filesystems fragment, but they _ALL_ fragment. People who tell you otherwise are clueless or stupid. The presence of software to defragment is nice and necessary for filesystem health. Get it. Love IT. USE IT. Profit.
This article is in response to all the clueless n00bs, drinking Apple's Koolaid and spouting ignorant stupidity as though it were lucid rational.