It's true that fragmentation occurs in OS X as it does in Windows, and therefore it's very easy to conclude that fragmentation must occur under both operating systems.
But the way both files are written negates the need to do it as often.
During the course of my career I've worked as both a Windows and Unix system Admin, covering all versions of Windows and Unixes such as Ultrix, BSD, Solaris, HP-UX, Linux and of course at home - OS X.
Boiling it down, Windows with NTFS or FAT has one major issue. It writes to the disk practically at the first place the head can write (in an effort to keep writes together) and has a terrible method for block allocation, not seeking out spaces of disk where the file can be written out in whole. It effectively "fills the gaps" between files, which is caused by and causes additional fragmentation.
When looking at it using a defrag program, it often looks neat and bunched up. And it's why even a month after a defrag, files appear to be still bunched up close to the spindle.
A month of solid use on a Windows system will show a measurable percentage of fragmentation, particular more so the larger the files are.
Fairly obvious even for the non technical. Rather than write as close to the last write as possible, even a random writing method would be better as it would cause much larger spaces to occur across the disk, and therefore much more chance of a space large enough to write a file.
NTFS is slightly smarter than FAT in that on write, it will attempt to find a space which is a good fit for the file, but this isn't very good for files which are growing at the time of write.
Bottom line is that Windows is quick to write to disk, but not terrible concerned with being proactive with allocation (FAT), or terrible at it in practice (NTFS). Fragmentation was never really a concern of these filesystems.
To be blunt - but honest, even the Commodore Amiga had a better method for writing out files back in the late 80's.
Most of the Unix filesystems (ZFS, UFS, ext4) have two major tricks up their sleeve. Firstly the writes are often proactively located in large spaces (i.e. not written close to last write), which in turn caused less fragmentation (because often the whole file can sit within the gap, or possibly only fragments into two chunks). Secondly, they often have delayed allocation which allows all the pending writes to be combined and written out together, rather than patch work them wherever there is space (like NTFS/FAT) - (there will usually be a Sync system call which will flush writes out to disk to commit them to the disk).
This is ideal for things such a long text logs or large chunks of data which are being created *now* but not yet needed immediately to disk.
It's fairly easy in practice to see the Unix style OS's beside the Windows FAT/NTFS and shake your head. Even in a month with less activity, the Windows filesystems will become far more fragmented than some of my even more thoroughly used Unix systems will be in a couple of years.
Back home on my desktop, I've never seen my OS X be more than about 1% fragmented. And even then, never seriously.
Fragmenting in OS X (and any Unix for that matter), is nowhere near as necessary as in Windows - where it's practically compulsory.