I've been playing with RC5. When I upload different cropped sizes of the same image, I am allowed to attach all of them to a particular post. What I am NOT allowed to do (unless I'm missing something) is to choose which sizes go in the thumbnail, gallery, and/or slider.
If I don't add a Feature Image to an individual Post, the largest size goes into the Slider. WordPress then automatically squeezes that image into the Gallery Image and Regular Thumbnail.
If I add a cropped Featured Image, whichever size I choose then goes everywhere including the Slider (which it doesn't fit). So whatever size I choose ends up being a compromise in the other locations.
The Media Library/Crop Image menu offers the option to crop a Master image for Thumbnail option only.....BUT for me......it doesn't work......it crops everywhere. I tried this several times with only two plugins installed (WordPress Beta Tester, and Maintenance Mode Plugin - All other plugins were disabled).
In a nutshell: Can the 3.5 Media Library/Crop Image menu be improved so that 2-4 cropped versions of a Master Image can be attached to each Individual Post, Slider, Gallery, or Thumbnail as the Admin User sees fit. Right now, it seems to be an All-or-Nothing choice.