"I think the best answer for this comes straight from this apple support KB
About optimization and fragmentation
Disk optimization is a process in which the physical locations of files on a volume are ""streamlined."" Files and metadata are re-arranged in order to improve data access times and minimize time moving a hard drive's head.
Files can become ""fragmented"" over time as they are changed and saved and as the volume is filled, with different parts of a single file stored in different locations on a volume. The process of collecting file fragments and putting them ""back together"" is known as optimization. However, if a failure occurs during optimization, such as power loss, files could become damaged and need to be restored from a backup copy.
Do I need to optimize?
You probably won't need to optimize at all if you use Mac OS X. Here's why:
Hard disk capacity is generally much greater now than a few years ago. With more free space available, the file system doesn't need to fill up every ""nook and cranny."" Mac OS Extended formatting (HFS Plus) avoids reusing space from deleted files as much as possible, to avoid prematurely filling small areas of recently-freed space. Mac OS X 10.2 and later includes delayed allocation for Mac OS X Extended-formatted volumes. This allows a number of small allocations to be combined into a single large allocation in one area of the disk. Fragmentation was often caused by continually appending data to existing files, especially with resource forks. With faster hard drives and better caching, as well as the new application packaging format, many applications simply rewrite the entire file each time. Mac OS X 10.3 Panther can also automatically defragment such slow-growing files. This process is sometimes known as ""Hot-File-Adaptive-Clustering."" Aggressive read-ahead and write-behind caching means that minor fragmentation has less effect on perceived system performance."
It's not that OS X doesn't need to be defragmented. It's that like Windows Vista and Windows 7, OS X automatically defragments your drive in the background when the computer is idle. You shouldn't ever need to manually defragment your drive in OS X.
