while some mac HDs may need cron script cleanups and cache cleaning etc (System Optimizer or Mac Janitor stuff), the speed boosts are marginal. in other words, they aren't really needed. as for defragmentation, i spent quite a while looking for a good defragmentor for my G4 Cube running OS X, until i realized that my performance had not decreased during my ownership of the machine. ie it had not become sluggish. after other people noticed the same thing, i stopped my search.
Windows screws up the HD by fragmenting over time, and while defragmenting may solve this in the short term, in the long term (6 months etc), windows becomes increasingly sluggish, despite repeated defrags. many of my friends here in my dorm (with windows XP) think they need a new computer because the speed of their machines has halved after a few months, despite repeated defragmenting. i told them to format and reinstall windows. it worked. machines were just like new.
the additional problems are that windows does not let you defragment some parts of the system, and also that windows generates lots of redundant Cache and Log files which take up lots of HD space and fragment the drive further. not to mention the files which mysteriously move around the computer.
while it makes sense that no 6 month old mac OS X install will ever be as fast as a new one, the difference in speed is extremely small. the time spent defragmenting, or backing up, reformatting and reinstalling, will probably be more than the time you save.
corrct me if i am wrong, but when ever i install a piece of system software on OS X, the last stage is called Optimization, and takes some time. it is my belief that apple is defragmenting the system software so that it is all in one chunk, to maintain system performance.
