Since you seem to feel that my polite request for objective evidence to support your position is unreasonable, and even insulting, I'll have to provide the evidence myself.
Defragmentation on HFS+ volumes should not be necessary at all, or worthwhile, in most cases, because the system seems to do a very good job of avoiding/countering fragmentation.
Fragmentation in HFS Plus Volumes
For these reasons, there is little benefit to defragmenting. ... If your disks are almost full, and you often modify or create large files, ... there's a chance the disks could be fragmented. In this case, you might benefit from defragmentation, which can be performed with some third-party disk utilities.
About disk optimization with Mac OS X
When recording a voiceover using iMovie an alert with the message "disk responded slowly" may appear. ... Frequent creation and deletion of files can cause disk fragmentation. This means that available disk space is not found in contiguous blocks. Although it's not a serious issue, it causes the disk to respond more slowly while you are importing media.
iMovie: "Disk Responded Slowly" Alert
If Disk Utility is unable to repair the disk, back up your important files, then try using a third-party repair utility that works with Intel-based Macs. You might also try a Mac OS X 10.4.x-compatible disk defragmentation utility to consolidate free space.
Boot Camp Assistant: "Your startup disk cannot be partitioned..."
So I repeat that from the evidence available to me, it seems that manual defragmentation is usually not helpful, but under some circumstances (which I've never encountered) it might be. If I were ever to encounter those circumstances, I'd choose to erase and restore the affected volume, rather than use a third-party utility.
