FWIW, this could be more than merely a marketing move on Apple's part. If you go back in time a few years, when NVIDIA first launched their 600 series, the hardware supported 4K@60Hz, but the drivers didn't. It took NVIDIA a *long* time to finally get the hack working that combined two virtual monitors into one screen. It seemed to take a lot of work, and other vendors had difficulties as well.
Everyone in the industry might just be waiting to do 4K@60Hz directly, without the two monitor MST hack. Note that this hack would only benefit the highest end late 2013 Macbook Pros - the ones with the the 750 - and I can't imagine there's a lot of volume on sales of that model. It may well be that NVIDIA simply declined to spend the effort needed to make a short-term Maverick MST hack solution for 4K@60Hz for such a small volume - and this might even be the reason Apple went with AMD/ATI GPUs on the Mac Pro - because they were willing to do the Hack.
Personally, I don't understand what makes using HBR mode in a straightforward manner as it stands so diffficult - the bandwidth should be there - but the industry's use of the hack means there must be some issue with it.
