@Leeghoofdyoure right, 5970, 6990, 590, 690, 760 Mars can theoretically run Stage 14, but they're completely crippled by VRAM.
It's just not a lot of fun benching those cards when no amount of OC will save them from starving on memory
Plus, seeing something a little more exotic like a Pro Duo in the charts is just neat by itself.
I'd make the distinction like ejunkie said, if the company supports it as a gaming/consumer card, it should be allowed as such. Something like Xeons are explicitly classified as server/WS parts by their respective makers, so they wouldn't be allowed still.