Diskeeper 2009 defragments drives with as low as 1% of free space, the 15% free space in the past was a Microsoft Windows recommendation because 12.5% that is reserved for the MFT and you can not move files into it. Each system fragment differently, so for an IT person that is in charge of a sizable number of systems would be a major headache to try to guess the best defrag schedule for all of them. It is highly recommended to have a fully automatic solution that would work in the background without taking resources from ctitical processes.  In addition, it is fragmentation that could cause the wear and tear on the disk than defragmentation for the simple reason that when a single file is fragmented into 1000 fragments, it would require a 1000 I/O requests to the disk instead of 1.