I don't pretend to know about everybody else, but when I need to see pixels, I tend to sit close enough where I can see the pixels. I normally use two or three 24" displays, each 1920x1080 or 1920x1200, unless I'm on the road. And I tend to sit about 12" away from the display I am focusing on. It's a fairly high-resolution setup, but I can still see pixels.
With 4x the pixels, I might actually be able to ignore specific pixels - alas, even Retina doesn't actually give 4x the pixels, not when comparing a 2800x1800 display with a 1920x1200 one, only a little over double. Better, but not there yet.
As far as pixel-accurate vs interpolated graphics goes, it would be a pretty poor "professional" program to which I provide a 1920x1080 image but it gives me a full-size preview using an upscaled low-res thumbnail. If I am displaying an image at 100% scale, it had better not be interpolated. If I'm upscaling from 1920x1080 to 3840x2160, the "interpolation" is easy to do without artifacts - just use four pixels instead of one, doubling both dimensions. If the program wants to apply smoothing, that's fine - as an option, since sometimes I want the actual original image.
However, a 30% upscale must necessarily introduce artifacts. An angled line that was smooth now has a jagged edge, since only 3 out of 10 pixels are doubled. Attempts at artificially smoothing these lines will affect the original image, which is why upscaled images often appear fuzzy. Either 25% or 50% will be significantly smoother than 30%, with an even 1 in 4 increase or every other pixel. It's not perfect, but at least it's even. Of course, many programs want to overly smooth the upscaled image even with a size multiple, and that feature may need to be disabled in some cases.
I guess the bottom line is that a Retina display is actually a higher-resolution display. It's not a fake spec, like 300hz TVs use, only allowing the source to provide 60fps. It does give a better image quality, and only interpolates when the image is lower-resolution than the display. If the display only accepted a 1400*900 image and upscaled it, that would be a different (and disappointing) matter.