It can be done, and in certain circumstances SHOULD be done. The notion that Mac drives don't get fragmented is nonsense. OS X does some defragmenting on the fly, but only on files larger than 20mb (if I recall correctly). I use TechTool Pro for this. I find it especially advantageous on our household's various Mac laptops: these tend to have smaller hard drives, and over time as programs are added and deleted, as well has files being added and deleted from general use, there's a performance drop. Apple would like to have everyone believe otherwise, but they're being disingenous.
On the bright side, it's required far less frequently than on a Windows machine. I probably defrag our Macs once a year, maybe a little more frequently when I know there's been a lot of adding and deleting. Windows is once a month.
I've defragmented Mac laptops for a couple of friends (machines that admittedly had too little remaining hard drive space, and which had been in use for a year or two), and the performance improvement was immediately obvious post-defrag.
