Well I thought the quip another customer made in store summed it up perfectly for me - he simply said this is 'the Emperors New Clothes' all over again.
If people are convinced they can see an appreciable difference then fine spend the extra cash. As I've said before eyesight is unique to the individual.
Even if it was a no cost option it really wouldn't bother me which model Apple dispatched. The standard screen blows me away as it is.
I have no quarrel with your statement that eyesight varies. But saying that the "emperor's new clothes" quip sums it up perfectly for you suggests that you think that any perceived improvement isn't real. That's demonstrably false. Put a Retina iMac next to an older, non-Retina display and zoom in on some text. The difference is plain to see, even to someone with aging eyes, and it's been obvious to everyone I've shown it to since acquiring my Retina iMac.
Now, whether that improvement is worth it to you is something only you can decide. In my case, I'm thrilled with the result. But implying I'm being convinced that I'm seeing something that really isn't there is wrong and frankly somewhat insulting.