Why is defragmentation unnecessary?
Solution 1:
The underlying filesystems used by Ubuntu, like ext2 and ext3, simply don't need defragmenting because they don't fragment files in the same way as NTFS. There are more details at ext3 - Wikipedia, the free encyclopedia
Solution 2:
Some argue that it's actually a myth that we don't need defragmentation. It's argued that we do in fact need it, but only once the filesystem gets pretty full (i.e. less than ~10% free space). Tools are available for defragging such as e2defrag.