I agree with your: "what is the point of buying a Mac and then triple booting OS X, Windows, and Linux?" But for totally different reasons.
I ask: why would you bother with even a double boot, let alone a triple boot? There is nothing you can't do with a Mac, in OSX alone, that you can do with any other OS.
First, hardware: Apple hardware is clean, reliable, with features that are difficult to find in combination on other systems. Apple hardware works; and when it doesn't they fix it. You don't have to keep fussing with it like you do if you build a machine from scratch. And its price is comparable to similarly equipped PC equipment (there was a recent post here on /. about that specifically). Yes, you can buy a PC for less... but that misses the point doesn't it?
As for the OS: OSX is like running Linux in many ways. It is solid, never breaks, it performs well, doesn't have virus and worm issues: basically everything that Windows isn't.
It is based on BSD, and has gcc and other open source tools. It has ssh, bash, tcsh, and X. You can build and run nearly any open-source application or tool.
What OSX is missing from Linux: fiddlyness. While running a Linux distribution feels good, at the same time it's a fair amount of work. Need to get a new piece of hardware working: compile a new kernel module, add that, and muck with configuration files in /etc. And if you're unlucky, possibly have to muck with device nodes in /dev or monkey around with udev configurations. And that's just one example. Every time you want to add or change something it's rinse and repeat time. Oh and forget Linux on laptops... it's famous for having spotty laptop hardware support.
But really the question is "why multi-boot"? With VMWare Fusion on the Mac, I really don't know. Just run Windows applications side-by-side with your Mac ones in OSX. Run an entire Linux development server in a virtual box. When you need to compare configurations, clone the sucker and try out a different one. When your Windows VM starts to get a polluted registry, slows down and starts to eat itself, delete and reinstall it...while compiling the Linux kernel in a Linux VM, while writing a software certification test proposal in OpenOffice running directly in OSX. No lost productivity simply because you have to reinstall Windows.
The real question here is not "Mac vs Linux vs Windows?", it's "why are you still multi-booting?"