I think it is hard to imagine how a 4K displays look on a 27" screen, plus there are other factors such as color, contrast, reflectivity, etc.. One can take a MBPr screen and imagine it being bigger, but that doesn't really capture the essence of it. Remember when the iPhone retina came out and people try to speculate how an iPad retina would look? I don't think one can get the sense of the latter by looking at the former.
For sure text will looks sharper, but most of all photos will show will a lot more details. I think it is hard for me to imagine anyone into photography would not appreciate more higher pixel density to view their photos. A print is typically 250+ DPI where as the current Apple Thunderbolt display is, I guess, less than 150 DPI. Sure, one sits further from a 27" display than one do viewing a print, but even at that distance, a higher density display will certainly make a huge difference.
I am sure many people who didn't think they need a 4K display will change their mind one they use one, just like our ancestors who didn't think they need fire until they tasted BBQ mammoth