It makes perfect sense that you want to block an accidental emerge -C @installed @world, but I really did mean it. I want to clear out everything (note: ROOT was not /) that was managed by portage so that I could go through the rest and see if there was anything I would want to keep. Simply adding "unmerge" to the sets' supported operations makes things work just great, so I don't think there are any technical reasons against this. Is there any chance of adding an option to force unmerging a set?
I guess you could sidestep the relevant check for ROOT != /