There's also the case where I was unable to find the source image and 1920x1080 and 1920x1200 were cut from a little different parts of an image, or for example 1920x1080 was downsized to 'touch the window from outside' as it says in MPC and 1920x1200 was simply cut from, say, a 2048x1536 image, thus both each wall had some data the other didn't. What I'm saying is, deleting duplicates is good, but if you're not sure then 2 similar walls is better(and that's where grouping would still be useful).
I don't crop my walls, fit to screen works flawlessly and I always get annoyed at wallpaper sites that have all the popular resolutions up to 4k, even if source is way smaller, I often have trouble finding the source because of then, so thumbs up for doing it client side!