i agree with haalp,, while the ranking system does have alot of flaws. kda would be a lot worse.
im currently ranked in silver 4 and i would easily rank myself gold 5 at my main. im still climbing but its so damn slow becuase i don't play carry champs so i need to rely on my teammate a lot more.
another issue with ranking system is while i may be gold 5 at my main.. im probaly around silver 1 or 2 at my jungle and around silver 3 as support. as ADC im probably like bronze 4. and top I'm probably silver 3 or 4.. so while i may be able to beat almost any silver player on my main. i cant instalock my main every game and expect my teammates to be forced to play champs they cant play. that would be like having someone trying to make me adc in a ranked game.
as for why going by kda is bad
as a support your not supposed to take the kills,, and as a jungler i like to give the kills to the laner that im ganking so they can win without me having to gank them again constantly.. also certain champs are "carry champs" which usually have a much higher kda