Ah,
Another victim of Microsoft Hype.

Yes, MS used to say that NTFS did not need to be defragged,
[in fact MS did not even include the APIs in Windows NT to allow 3rd parties to defrag NTFS filesystems].

But in more recent time they have conceded that NTFS needs it on a regular basis - this is in part a inherrent design limitation of the filesystem since it was based in part on the VMS Operating System and its filesystem structure.

Hence their inclusion of the Defrag utility in Windows 2K [and Windows XP.

More to the point - you should generally not fill a NTFS volume beyond about 50% capacity as you see quite dramatic slowdowns once you go beyond 50% full on NTFS volumes.
This is due to the fact that NTFS leaves deliberate file growth gaps when the drive is being filled up with data.
By the time 50% full is reached NTFS then begins to fill in the gaps it left, reducing the room for existing files to grow contiguously amoungst other performance problems.

How well a defrag utility will work on a RAID 5 disk volume I am not sure having never tried it.

I can suggest that it will generally be a slow process as RAID5 disks are optimised for more reading than writing [since the parity calculation and then striping of parity across multiple platters take time] and as a Defrag involves lots of writing to move stuff around you may find it something you do over a long weekend once in a while rather than a regular thing.

BTW: For those who are interested there is a small VBscript file you can obtain that lets you run an unattended disk defrag using the inbuilt W2K/XP defrag utility which is normally interactive only. This can save the need to buy a unattended disk defrag tool as NT already has everything needed to do this.