Too bad none of the MacBook or MacBook Pro displays can show a 4K video at full resolution. I was thinking of upgrading my MacBook Pro this year as I'm still using my first-gen Retina model from 2012, but being able to play these heavy 10-bit 4K H.265 files is almost pointless when I can't use my 5K iMac as a screen...
For me it's about compatibility. If I happen to have a 10-bit 4k file, I want to have the flexibility of playing it anywhere, so I tend to make my purchases strategically this way. And I also want the ability to multitask with no fan noise while playing a video.
It's similar to back in the day when h.264 was just coming to the forefront. I wanted h.264 hardware decode capability for my 720p MacBook Pro even though Core 2 Duos of the time could decode 720p h.264 in software, although 1080p was sometimes a problem. Having hardware support meant:
1) No problems playing 1080p video, which was becoming mainstream, and no need to transcode down to 720p to get decent playback. 1080p sometimes did work fine in software, but more often it did not.
2) 720p video was never a problem. 720p did work in software, but in some cases you would still get occasional stutters.
3) No fan noise with h.264 playback.
4) Fluid multitasking and fluid menu activation when watching videos.
It's just so much more pleasant surfing on my 2.26 GHz Core 2 Duo MacBook Pro with hardware h.264 acceleration than it is my wife's 2.4 GHz Core 2 Duo MacBook with no hardware h.264 acceleration. (I bought the latter used as a cheap kitchen recipe machine for the wife.)
The same applies here to a certain extent. Even if your machine can play some 4K 10-bit files in software (eg. 4K p24 with low complexity), it usually needs to do with high CPU load, poor multitasking ability, and loud fans. And even if it works on an iMac, it may not work on a MacBook Pro or a MacBook. OTOH, if you have hardware support for all of the above, then no problem.
Even iPhones can do it these days, since they have hardware support for it. I think that too is a pretty good indicator which direction Apple is going.
It's a shame Apple got rid of Target Display Mode.
Agreed.