You don't need defragmentation software, as the file system Mac OS X is journaling, thus taking care of fragments itself.
I once did use iDefrag to scan several of my internal and external HDDs (and some even had journaling disabled), which have been used for years, and guess what: none of them needed defragging, even though I copied and deleted a lot on them.
Journaled file systems don't require defragmentation it does nothing to them, file system like NTFS are journaled and don't have any other methods of looking after themselves, so they need defragmentation. So Mac's that only have a HFS+ Journalling Partition don't need defraging ever.