Discussed a million times, but it's very very difficult to make it accurately from statistics only. If i have a 0/10/41 Soraka game, but my ADC is 22/1/4, then was i a bad support? Depends. All those deaths were made so the carry can destroy the enemy team, or were they free kills.
If i get focused like hell every single time, but survive long enough to win the teamfight, then my death worth the price. If i survive the teamfight, but give but all i deal minimal damage for possible assist, then run away, then i was a worthless player. Which one gives the better KDA?
Also the ratios are hard to put in. If you put too much value for objectives, then people start ignoring the team to get more objective. If you put too much on the KDA, then people start going for kill instead of focusing the objectives. I HATE it when enemy Nexus can be hit, and teammates start farming, or hit enemies on start. Instead of literary winning the game.