Of course, there's always the question raised by prospective users: Why use Linux at all? OS X has Open Darwin at its core and the Mach Unix kernel as its engine. It's a valid question, and the answer depends on what you want a computer to do for you. OS X has Unix as a foundation, that's true; but it also pays an operational price for ease of use.
Tech note for you geeks out there: The Mach kernel used in OS X uses caches in ways that can cause heavy use of the memory bus to make up for data that isn't available to the processor caches. For example, Linux makes system calls directly via a register file. Darwin makes them through a memory buffer, which is then written back to the register file after being bumped around. Although this allows for easy software modularization, it negatively affects execution speed. It also means that just throwing more processors or faster processors into a system won't break this bottleneck because it depends on contention for the memory bus.
There are users who want all the power they can get out of a machine, and some OS X internals (such as the one discussed above) blunt that raw power. Linux gives you the ability to mess about with the OS that is being executed in a way that OS X doesn't because OS X also has a proprietary way of running on Apple's hardware. That's exactly what OS X is designed to do: Even with its open source roots, it uses Apple-brand drivers and hooks in applications (Aqua, Cocoa, and so on) that are not open sourced to get things done. If you just want a really fast server on network, you might want to remove the overhead imposed by OS X that you won't use, anyway. Linux allows you to get OS X out of the way of your machine's hardware. You do have to be able to talk to Linux to get it to do what you want, however. And not everyone out there can do that. It's not that hard a skill to learn, by the way, but there is a learning curve.
I'll bet that the main reason for using Linux is actually fairly simple: People have Unix software (like LaTeX) that they need to run for work. Linux makes using (and compiling) these programs fairly simple. OS X may need far more tweaking because it is not pure Unix. On the other hand, Adobe Acrobat isn't yet available for the PPC flavor of Linux (although there is a program called xpdf that can read some PDF files), so it really does depend on what you want your machine to do for you. And running Linux is safer than running a proprietary version of Unix in which the vendor might disappear on you. The Linux movement has gotten too big to evaporate.