I suppose we deal with a similar situation now though....on a standard screen, you really don't know what it will look like on print by viewing it on your monitor. But you know it will look good on other screens.
With the retina, it seems you might actually have a reasonable view of what it might look like printed. But you lose all sense of what it will look like on others monitors. (at least that's my assumption).
To be honest I get a darn good idea of how my prints will look when compared to the monitor. Since I use the high-res display and the Thunderbolt Display, it's pretty close to print as far as I can tell.
As for the issue which I previously mentioned, I think you misunderstood. The issue I bring up is one of working space.
Let's say that you have a 15" uMBP with a normal screen of 1440x900. When you open Aperture, your working space will be around 1000x700 and your photos will be downscaled by Aperture for display within the working space. Let's say you're shooting with the 7D, so you have a 5184x3456 photo which is shrunk by a factor of ~5 for on-screen viewing. So when you tell your loupe to view an area of 50x50 on the downscaled photo within the working space at 100%, you're selecting an area of the actual image which is 250x250. The loupe will therefore show a 250x250 image.
But now on the rMBP with a 2880x1800. The working space on the Retina Display is then 2000x1400, and the photo is downscaled by a factor of 2.5 instead of 5. When viewed side-by-side with the 15" uMBP without loupe, both images should look identical, with the extra resolution of the rMBP making the photo appear sharper.
Now apply loupe to the rMBP. Tell it to view the same apparent area on the rMBP, which should be an area of 100x100.
The area of the actual photo which you're telling Aperture to display is still 250x250. The loupe will
still display a 250x250 image on the Retina display.
Now, we know that the pixel density is double in each direction on the rMBP when compared to the uMBP, right? This means when two images of the same resolution are displayed on each screen, the rMBP will be 2x smaller in each direction as the uMBP. So the 250x250 image of the loupe, when displayed on the rMBP, will look
tiny when compared to the uMBP's loupe. Increasing the magnification of the loupe will cause the image to be blocky and even harder to view than before!
This issue of smaller loupe personally would bug the heck out of me, since I use it all the time to inspect and refine an image.