However the fact remains they cannot keep supporting new graphics cards ad infinitum.
What do you think the rest of the Mac line up uses for graphics? Tiny Oompa Loomps running around on a treadmill ? Largely the same basic graphics architecture updates that are going into the mid-high range mainstream desktop cards is eventually going to go into the mobile versions (if not concurrently). Evolving graphics architectures have to be supported over time or else graphics is dead on the Mac product line.
In short, even if there isn't a new Mac Pro to attach a new graphics card to Apple (and the GPU vendors ) need to keep the driver updates going. If they only work on it when about to release a machine they likely will loose team expertise over time (folks will go off to other projects during lulls ) and/or won't be able to ship without an extended delay when new GPU technology is released. Neither one of those is a good idea for the Mac line up in general or the Mac Pro specifically if want to be competitive in the modern personal computer market.
he Titan card has PCIe 3 capability so eventually the Mac Pro will refresh.
The entire rest of the Mac product line up is already on PCI-e v3.0. So does the other drivers that have been released already for 2012 era GPUs.
Specific GK-100 work in an OS X beta or release is indicative of a new Mac Pro card since the architecture derivative optimized for workstation/server GPGPU work probably isn't going to evolve into one that will move "mobile"/"mainstream over the short term. A several years from now though the "mobile" GPU that some folks will sneer at being in an iMac will be the equivalent of a Titan. ( maybe not all the double precision throughput but certainly on the single precision side. )