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It may not be in AMD's/NVidia's interest to spend time working on cards that Apple isn't shipping, but they sure seem to be doing it, as evidenced by the wide range of cards they're supporting these days.
They'd be suicidal not to be expanding the card options right now.
First, Apple has totally screwed up the Mac Pro product management. There is a substantial opportunity created by Apple falling on their face to sell cards to those "stuck" on this basic 2009-2012 model to significantly move forward on performance based on buying PCI-e GPU card upgrades. Apple selling cards from the 2009 era only opens the door for others to fill that gap.
If Blackmagic and Nvidia can walk in and say
" ... DaVinci Resolve, colorists seek the highest performance possible from their systems, and with just one of the new Kepler GPUs our users will be able to work with 4K imagery on their Mac Pros in real time. ... " by dropping several thousand dollars on their solution to make a current Mac pro a real-time editing bay then why not. If Apple gets screwed for the next couple of years until that user has money to pay for a new base unit then that is Apple's problem. Apple snoozed and lost.
Second, they have to stay in the game to be eligible to land the job of being selected the default for the next Mac Pro. Yeah the other Macs tends to all swing the same way ( all AMD or all Nvidia) this is the one Mac those vendors can keep a foot in the door even if they loose the much larger contract. Given Intel is increasing taking a larger share of the graphics work home, being picky isn't an option. They can keep a subset (if not all) of the driver team active just with filling competitive gaps on Mac Pro if necessary. The expertise on a Mac Pro oriented driver could be used on next's mobile driver also. There is no huge long term downside in staying in the game if Apple is going to continue to buy significant numbers of discrete GPUs.
Everything Apple is doing right now, or lack of doing like shipping, on the Mac Pro is indicative that they "mothballed" the Mac Pro team and now are stumbling around trying to get it restarted. Keeping a team active, even downsized slightly, is generally more competitive than doing what Apple apparently did.
Just because Apple chose to screw up the Mac Pro as a platform for revenue growth doesn't mean their partners need to do the same thing. In fact, taking advantage of Apple screw ups is generally a good place to make money. Apple does screw up. This sub-market is one of the major instances over last 6-8 years.
