To me the loupe is very important. The details are a very important part of the quality of an image.
Some part of the image will NOT be visible in the green box. Consider this an anti-theft feature. You cannot stitch together the parts and get the whole image.
I have some gripes about it though:
Very compressed. The quality often looks poor because of compression artifacts. This is my major gripe.
For very large images, it is not always full resolution. I do a lot of stitched panoramas, so this is where this problem tends to show up. This is a big problem, in my view, also.
But I would much rather have it than not, because you really can't judge the resolution and quality of a photo without a close-up inspection, and the loupe provides this function.