Defragging reduces the number of read/writes
Defragmentation makes sense on a hard drive with hundreds and thousands of files that are constantly created and deleted. Because there are no read/write heads on a flash drive, even if a song file got fragmented, there is an imperceptable lag between reading contiguous data chunks, and there would be virtually no performance increase.
Also, because flash memory longevity is largely based on the number of write cycles rather than read cycles, one could argue that routine defragmentation, which would actually generate a larger number of writes, might adversely affect flash drive lifespan versus not defragging at all.
Steve