Due to how flash memory works there is little or no point in "defragging" them. Cleaning them by deleting everything and starting again can help but that is it, they cannot be sped up by defragging as the FAT layer seen by the OS is by no means a real representation of the physical media. You can make it look pretty on a graph of disk usage but the file will still be physically fragmented on the media... the other point is that without a physical head reading the data there is no seek penalty for a fragmented file.
See
http://www.aximsite.com/boards/showp...2&postcount=12
As Speedy says though, it is useful to hard reset as it will remove rubbish that may have been left behind after an uninstall that could be slowing the OS down.