I never thought about this! If you are bad at a role, and you are a diamond player, but really a gold adc, then you should not play with diamond adc players. I do think you can be skilled without mastering all of the roles, but you become a more skillful player as you play more roles in my opinion. The problem is obviously the people who want to play more than one or two roles, but also want to climb effectively, and that is where i think the opt-out works for all of the people. Someone like you gets to have role-ranked and not drag down their team when they get auto-filled to adc, but good for someone who plays every role also, since they can simply, opt-out.