In my opinion it is necessary to request a minimum of normal games won, as I said before, they can't play ranked queue with 100 wins in normal, because that means they got to level 30 playing custom games vs bot or vs AI.
Consider that a person with less than 200 normal wins, I get to level 30 or just playing custom games vs AI cop, where the difficulty is minimal, the bots do not use the grass to hide and throw skills, bots don't go after a wall and use flash to escape to you, bots don't wait in the grass in front of baron to expect the enemy team to force a teamfight, those things I think you learn in 5v5 team, or in normal, where the vast most people respect the "betagame" adc + Support bot, Ap mid, Top and Jungle Fighter.
Isn't arbitrary, is necessary.