Why do we expect that playing a fake version of the hero against other fake heroes is better training for playing the real game than just playing the real game? What if the player picks up habits that are only good when the opponent cannot use the correct runes, spells, and masteries? In our Street Fighter example, people could learn to do things that Chun could punish with SA2 but not with SA1, and they would get away with them until their summoners reached the max level.
What if the player is an experienced LoL player who moved to a different continent, or a genre savvy player, or a LoL player whose previous account was banned? These players can't really benefit from the forced grind, even if it has any benefits.