the reason this idea works in WoW and not LoL is because WoW is scripted encounters, to be successful in leaheu you need to be very adaptive, if you are comfortable in any situation your elo will rise compared to someone who can only do a few things.
There are too many factors to have certain roles, sometimes a certain pick or ban changes the entire strategy for the team, what initially was a team fight comp suddenly changes to a poke comp because poke became a better strategy to use.
I've had games where we started our picking janna graves jayce, after seeing the enemy picks, we changed our entire strategy and picked taric nunu, running janna in a 1v2 scenario, jayce mid, and 2v1 top, top lane, the enemy team was unable to adapt to the change, they anticipated a strategy and we capitalized on it, this kind of move simply doesn't happen when you force roles in the way you describe.
I recently had a game as amumu, I planned on running Azingys build(3x dorans sunfire abyssal), 5 minutes into the match I realized my strategy of attacking there carry with that damage build would not be the best for that game, I switched to triple gp10 suppport style amumu and we won the match, having set roles does not work, you have to be fluid as a player and determine the best strategy without set "roles" other than the role of contributing to the win