My apologies for tackling this one sentence at a time (and slightly out of order):
He claims that daily defragmention does more wear and tear on a hard drive than say doing it on a monthly basis.
Lets say you have a drive that has 1000 fragments of files on it at the 1st of the Month, and normal file operations are likely to create approx 33 new fragments each day, ending up with another 1000 fragments by the end of the month.
If you defrag on the 1st day, then wait to defrag again on the 30rd day you will end up with a total of about 2100 fragments that will have to be dealt with overall. Where did the extra 100 fragments come from? They were already there, because there are always some files that cannot be fully defragmented and the count of those fragments are left over to add to the daily count of new fragments from file operations.
If you defrag on the 1st day, and each day thereafter thru the month, you will end up with a total of about 1700 fragments that will have to be dealt with overall. How is that possible? Because over the course of the month as you reduce the amount of fragments that build up each day, you also reduce the rate of fragmentation because there are more contiguous spaces available for the OS to create new files or updates to existing files within. More contiguous files also means less activity is needed to read the file table looking for fragmented files, and to calculate the amount of fragmentation and where to move fragmented files to. This can even serve to counter the extra build up caused by files that cannot be completely defragged.
(The figures above are made up, but the reasoning is sound imho).
I tell him I set jkdefrag -a3 to run automatically daily on all the computers I work on.
This means you are combining defragmentation with file optimization tasks. The file optimization pass can create extra fragmentation, but it also tries to place files in better positions for frequent use, which is hoped to reduce wear and tear by
making file operations more efficient overall.
Do you agree that defragmentation/optimization over the lifetime of the HDD causes less head movement than if the HDD was either neither defragmented or only on a monthly basis compared to a daily basis.
Assuming that the usage pattern is the same in both cases, and usage is heavy enough to need regular defragmentation
...either never defragmented: Definately will result in less head movement on defragmented drive.
...or only on a monthly basis compared to a daily basis: I would say a Daily defrag will be more efficient that an monthly defrag, the question that was not asked would be if a weekly defrag could suffice in the place of a daily defrag for most file operations.
Also if you have multiple drives/partitions you may find some drives (system and regularly changing data) require daily defragmentation, while others (storage and archive data) may not require much defragmentation for months at a time.
I hope this helps.