If you work for a film studio, then I can understand blowing 3 grand on a machine. But most folks I know are buying these to run photoshop and illustrator, then claiming that a PC simply would not do. My POS laptop runs these programs fine, and my $750 new PC box runs them like lightning.
Running OS X with (mostly) no major issues. Given that most software is available for at least two operating systems, it's really down to choice these days. Sometimes, sentimentality and/or perceived value overrides rationality. I'm pretty sure this is OK and not a major flaw in humanity, most of the time.
Bootcamping - go ahead, try that on a Windows box. I Bootcamp into Win 8 and OS X. After a few driver updates, Windows 8 performs flawlessly (aside from, you know, being Windows 8). If I really felt like it, I'd run Linux as a third option, but it's way more convenient to run it as a VM.
People get way to wound up about the hardware/OS choices that other people make. Really, aside from having to do support for them, it should probably only be about as controversial as hair length.
The only thing I can possibly imagine would be some seriously high-end 3D rendering or massively parallel simulation software.
To the OPs point, there is zero reason to get the Mac Pro over even a Mac Mini.
Seems like there's a hole in Apple's lineup. You can't get a discrete GPU without a monitor attached (iMac) or spending >$3000. Not that it matters much for Illustrator, Photoshop, etc. which are not GPU-parallelizable anyway. At least not yet.
They aesthetically please the eye in clean, minimalist offices.