Your average user shouldn't need to defrag, though that's quickly changing in my experience. Mac OS X defrags files under 20MB automatically. Anything larger than that would result in lessened system performance as the OS constantly trys to defrag larger files. That said most people don't deal with a lot of files over 20MB, sure the occasional video files or imovie projects but nothing major.
I do some heavy video editing as well as use multiple virtual machines in Parallels, I can assure you defragmenting every so often helps me out. The best tool I have found to defrag is
iDefrag. Not Norton, Not Tech Tool, Not Drive Genius. They seemingly have mastered the art of optimizing my drive and they have a neat free utility to make a bootable disk(make sure you have enough RAM).
Is defragmenting neccecary. No. Especially not in the way a Windows user would be expecting.
Is defragmenting useful under certain situations. Yes. Even then is it neccecary. No
Will we eventually come to a point where it will be neccecary. Maybe?
As photo files become bigger and more people use imovie and other large media sources files over 20MB will become the norm and not the exception. However by that time I expect drives and processors will be fast enough that Mac OS X will be able to defrag 40 or 60 or 100 MB files by itself with out performance hits.
If you are dealing with large files like I do I think you will be pleasantly pleased in performance by iDefrag.
***EDIT***
Nothing really to do with the topic but Diskwarrior is the greatest utility ever. I've seen it save more butts than Sir Mix A Lot could shake a stick at.