Well i think the reward should be that in the next ranked game you get a free dodge during champ select, and you can continue to dodge until you get into a ranked game.
Another reward i was thinking about is you can also select your pick order. If more than 1 players on a team had S or S+ from previous game the game randomly select whom to have pick order selection.
Yeah, couldnt that be a good idea?
Huh? You only get S or S+ if the 4 other people on your team did a bad job and you really powered through. If all teammates on your team are doing great and its an easy win in 15 min then you dont get an S because then its too easy.
In diamond and such ranks people also tend to be more careful about not dying meaning it gets more difficult to get S or S+.