I'm going to repeat a request I made long ago. Never did hear why it wouldn't be useful or very hard to implement. A simple "do not clear render output" checkbox. One would do a crop and with that checkbox checked, make a change like to a surface shader and then do another crop near or next to the first one and then render again. And then, if desired, do a third, fourth, etc. Much quicker and easier than saving off crops and comparing each in an editor.