Champion performance rating mostly only encourages kills.
At first I didn't care, but now Hextech chests on the line it's annoying me to see those guys with 20+ kills get S(++) ranks most every single game. Not necessarily for performing their role, or even in many cases for even doing the most damage, just their scoreboard numbers.
It feels like as a support, as control mages, as tanks - there's nothing to do to get S ranks, some games I can do lots of damage and have excellent contribution but kills will be low - I have to play an assassin and kill 20 people, even if I lose I get an S rank, it's just not right.