Disk deframenting is not as important as it used to be on Mac OS X. But it is not unnecessary.
I was doing some periodic DiskWarrior maintenance on my computers today. My laptop has 10 GB of free space, out of 55 GB. DiskWarrior tells me, OK, it is ready to replace the directory, but it doesn't have 276 MB of contiguous free space to fo a "fail-safe" directory replacement. Yow.
I should really buy TechTool Pro, but it is so expensive. $100. I am checking out iDefrag and Disk Defrag.