""As for efficiency and performance, frames are packed into texture atlases when exporting anyway, so delaying the processing or not should be negligible.""
True but now C2 is importing a sheet, slicing, and then putting it all back into a sheet on export, so why bother XD; All this really does is give you extra space for image points and let you crop...meanwhile drastically increasing preview times and bloating your project with thousands of images & folders.
It makes the editor architecture simple. If the editor dealt with spritesheets and you edited a single image to be bigger, it could cause a cascade of spritesheet repacking, which affects many images instead of just the one you edited.