It's not just 5k, it's retina screens in general. Almost every new smartphone and tablet uses a retina screen these days, and therefore should be taking advantage of high resolution images.
Even on my iPhone 6S Plus, if I go to a site with non-retina images it's easy to tell how unpolished they look. Re-load that same site with retina images, and it looks sooooo much cleaner. So, it's not just on a 5K iMac, this happens on any screen which takes advantage of a very high pixel density.