Well - clearly I have a different interpretation of what is legitimate negative feedback - but recently was told that if a player A does not recognize player B as a superior player - then player B can file negative feedback on player A - and it is legitimate..
So all you ego maniacs out there - get your negative feedback button ready. You now have a legitimate reason to use it: 'Your ego is such that everyone who does not recognize you as a superior player deserves negative feedback'