5K Monitor direction spells redundancy for my 8 month old Mac Pro

[calaverasgrande]

I can think of a few things...

Working at 5k lets you have some on screen chrome around while you preview your footage at a native 4k. If you spend all your time working with video full screen, yeah, it's pretty meh.

A lot of other developers (application, web, etc) also need to work at Retina resolution but don't want to sacrifice the current effective resolution they work at. 4k would be an increase in quality, but a drop in working space at 2x DPI. So 5k is a big deal for that.

except in video post we pretty much always use one display for timeline/tools and another for viewing the program material. Cramming it all into one display is far too crowded. It would limit you to a very short timeline with tiny previews.
It really does just boil down to retina.

Though I think for most Mac Pro owners Retina is probably less of a priority than sheer desktop real estate.
As far as smaller retina displays that D300/500/700 can drive, I think those would be acceptable for folks in post or some graphics professions.
Just imagine two across and it is plenty of space.
For a laptop adding a 22" or 24" display is perfect. Much larger and it makes the built in LCD seem too small.