Macs do not require frequent disk defragmentation, mostly because HFS Plus has a feature that does most of the work for you, behind the scenes. 😀
However, this feature of the file system does NOT defragment huge files (100 MB or larger, I believe), so if you have lots of these investing in a defragmenting program MAY be a wise choice. Also keep in mind that, if you clone the internal drive with CCC or SuperDuper, boot from the clone, then clone the clone back to the internal, as a side effect both drives will be completely defragmented.