I think that there are enough glaring problems with the system for people to not take it seriously as a skill metric. For example, I went YOLO Cassio top last night against a Sion and lost my lane pretty hard. I over-pushed lane WAY too far and too early trying to build passive stacks, so to farm anything I had to be extremely over-extended. A roaming fed Sion was probably the biggest contributor to the loss. In other words, I single-handedly lost us the game since our other laners had good advantage until Sion showed up.
But I ended up with the highest score on the team. It was a B or B+ when it should have been a D. It's just all based on weird metrics that don't account for true impact on a game. Another example was a game where I was basically a meat shield support, and we ended up with a super-fed ADC while the other team had a fed turret. I got a C that game, even though 3 or 4 of my deaths were executes that gave our ADC a kill.