This is urban legend. CPU is bigger bottleneck (esp. in games) than PCIe in MP 1,1.
Much better OpenGL and OpenCL performance, lower heat emission and power draw, triple displays support, better reliability of 5870 aren't worth it?
I'd rather say that if you're a gamer, owning MP is not worth it
I play video games on my Mac Pro 2.1 and its worth it to me I can play Battlefield 3 at Ultra Settings with no issues. Mac Pros have a lot of power they are worth it, not to mention now the ability to practically run Mavericks.