Well it shows reward for red star rings than just for achievements. If red star rings or S-ranks offer more rewards I'd be more interested in earning them.
I believe SEGA realizes this too... But whether they'll still use it on later Sonic games that's another matter.
Oftem to me a B-rank is a good enough for me and move on to the next stage in order to enjoy the story.
I'm already satisfied with D level if the level's frustating to play, lol, like one stage from Sonic Lost World (3DS) with suicide bombing penguins that always respawn. Not to metion that annoying Snowball of Doom.
But getting red rings from each level to trade it with rewards sure does add replayability a lot for the game.