"... had a user ring up (legitamally) complaining of lack of disk space. Helldesk response - run a defrag."
I used to work on a system on which that might actually be good advice.
GCOS (Honeywell's mainframe O/S) had the concept of "temporary files", which would automagically go away on logout. Being temporary, they didn't count against your disk-space quota, which made them essential for users with tiny quotas, e.g. undergrad course accounts, to be able to get anything done.
The thing is, the file system was extent-based, and temp files could only have a very limited number of extents. As the file system became more fragmented, the maximum attainable temp-file size got smaller and smaller, to the point of being unusable for some purposes. Scheduled down time to defrag the disks was an every-couple-of-months maintenance task.