The OP is actually correct when it comes to "consuming content". If you bought an iMac to watch movies and play games - you really don't need 5K. In fact, in some scenarios, the non-retina iMacs will actually be better than Retinas (watching Netflix, for example). And even when 4K becomes more standard, to be honest, watching a good quality 1080p movie and a 4K one isn't all that different from one another when viewed from a normal viewing distance (it is noticable, but nothing dramatic, in my oppinion).
Even while looking at large resolution photos - the difference is not that big as people might expect (mostly due to how our brains work). Sure, there is a difference, and professional photographers will see it - but for average users it is not that big, really.
So, is the 5K Retina only good for 4K video editing? No. While you might not notice the difference when just
consuming content (again, mostly due to psychological reasons)
creating content, any content, in 5K is very different. When you edit something, when you illustrate something, when you focus on your work as you always do while working as opposed to consuming - the difference between retina and non-retina is, well, dramatic to say the least. I am an illustrator and drawing on the Retina iMac allows me to see my work with all the details without zooming (or with very little zooming). This is really important, as it is really hard to get 'the full picture' otherwise. Even if the image can fit the screen zoomed, the larger it is physically, the harder it is to get a good "feel" for it. Retina is a game-changer in this regard. In fact, I know several artists (like the well-known Skottie Young, for example) that are switching from a Wacom Cintiq to a Retina iMac + Wacom Intuos just because of this difference.
So, in short - if you use your computer for consuming content, yes, the difference is not that big. If you're using it to create content - any kind of content, not just video - then it's miles apart. Choose your iMac according to your specific needs