well the issue is until you get crit you do no damage.... so by the time you get your
you have no ad to trade or farm with. no attack speed.
adc needs a mix of attack speed, physical damage ( AD ) and crit
crit just amplifies the physical damage you do to a target. lets say you do 100 damage to a champ after reduction from their def stats and if you crit you will do 200 damage ( 200%) and with IE you will do 250% damage so 250 damage
but if you have 50 ad and crit you do 100 cuz you have no ad ... the more ad you have when you crit the more damage your crit does