Mac OS X is based on the Mach Kernel. Other parts of the OS come from NextStep, FreeBSD and NetBSD. All of which are mainly Unix-based. Thus Mac OS X has gained, since OS X 10.5 Unix 03 certification.
The point being? Unix/Linux desktops and servers do not really experience file fragmentation. And certainly not on the order of Windows-based systems.
You may very well defrag. a Mac OS X file system but it is not a necessity.