The Grade System Encourages Noobs to Play Passive
I notice wayyyyy too many noobs just playing passive during games to pad their KDA...when the game is going badly instead of trying to make a play they sit back and play safe the entire game. Case in point just got out of a game with a Malphite...this guy was tanky as hell, but he would never go in. He ended up getting an S-, when it was undoubtedly his "safe" play that lost us the game (we never had a tank).
Personally I don't think you should get a grade...your only "grade" should be if you won or lost. Pass or fail. I'm sure that Malphite felt pretty good about his S-, but the playstyle he used to get it did us no favors, and is ultimately teaching that noob the wrong lessons, especially on a tank character that needs to initiate fights.