Better form of debate than the OP, ill give you that. But avg 8 kills per game, isnt enough of an average to say that he is the reason why games are being won, high win rate yes, but thats because most teams dont know how to handle back-dooring, which with 2 (or 3) ADC's should be a major strategy pull. Or, teams are having issues dealing with 2 (or 3) ADC's, both are pretty good probabilities. Short version, 2 ADC's is still a pretty new meta, and its only good because its new, once people figure out how to deal with it, it'll probably fall off in effectiveness.
Top lane - Although he does counter most top melee top laners, there are still a good chunk of top laners that can easily beat a ranged champ in a 1v1 as long as they play smarter. Most top laners i've met cant seem to figure out how to deal with ranged tops because they're so used to being melee vs melee that they just complain how OP it is, then feed. So its not Graves in specific, its across the board for all ranged vs melee in Top lane. People dont wanna learn how to fight vs ranged as melee, so they just complain its OP and whine for nerfs.
Jungle - Actually havent tried him there yet (I'm a jungle main) but although his clear time is probably through the roof due to his AoE AA, he has no real good form of CC, and as long as some one uses wards, and doesnt over extend, graves shouldnt be able to get in close enough to actually make a solid gank, I know he has his dash, which helps a lot for positioning, is also dependent on how much attention people pay to the map, and how safe they play. (Correct me if im wrong here) Counting Bronze/Silver into the formula (and probably low gold if my understanding is correct) Really skews the numbers IMO, since most people of that skill level barely understand what wards are, let alone map awareness or playing safe. It doesnt matter if they're being camped and know it, they push right up to enemy tower, get ganked and die. And some how think its not their own fault in the process. (Topic for a different conversation) So Graves being good in the jungle is more due to player error, than graves being really good. Having awesome clear and burst are important, but the skill of the people you're going in on have a huge impact on how well your gank goes off. If a champ has strong ganks, you should play safe and not let them get as good of a gank off, but alas it doesnt work that way.
ADC - Not much to say here, the only ADC's i can play are Ashe, and Draven. Lol I know graves has always been a good pick as an ADC as long as you have a tanky support with a stun.
TL;DR - Its not Graves thats the problem, its the player base not understanding how to work with more than one ADC on a team.