With no intention of jumping into the argument in question here, I have a slight issue with your definition of a gamer. I'm an intermediate photoshop user, web designer, and gamer. I don't just use my computer for games or work, there's this huge gray area in the middle. For me, the Mac Pro is the best of all worlds. I wouldn't dare rely on Windows for my workflow, design, and productivity software, OS X is a must for me. However, the ability to duat boot into Windows and play games natively is a bonus, one that I'm willing to pay a premium for, and whether or not it's even a premium is up for debate. Sure, I could build a PC just for games but if I can't run OS X ever then that machine is useless for me.
I'd be surprised if there weren't many more people out there who welcome the power of the Mac Pros for work and play, recognizing of course that the majority of buyers will be professionals.