Depends on who you ask...
I asked this question many moons ago, was told "you don't need to defrag Linux", and I was given a link to a site with an explanation. The crux of the argument was that Linux is a multi-user operating system, and that nobody can predict what requests multiple users will make at any given time, thereby negating any benefit from having a defragmented drive. I agree, but there's an underlying assumption that multiple users will be on the machine at the same time. That's not the case with my computer at home. Daemons might be considered other users perhaps, but for the most part, my drive remains quiet unless I open a new application, file, or something along those lines.
In the same thread, someone said that fsck will tell you how fragmented your drive is (I think it reports it as a (non-)contiguous percentage). That user was reporting something like 30% fragmentation.
It depends on what you're using the computer for. If you plan to run a server of any kind, then you cannot anticipate read/write requests from users that are logged in, meaning that fragmentation is not a concern. If you are the only user on the machine, defragmenting a drive can
help by reducing the number of jumps the drive read/write heads have to make to get all your data.
Disclaimer: I'm not an expert on the internal workings of filesystems, but unless there's some sort of "cleanup" overhead associated with a filesystem, then a drive will become fragmented over time. If the cleanup overhead does
exist, then essentially the filesystem defragments itself in small bits every so often.