Ashe Passive UP

Is it just me or is ashe's passive complete crap, woopty freaking doo a free crit after its done stacking which is quite a while. When you are farming in lane her passive is useless it seems like. Because she can't stack her passive if she is constantly last hitting. I think there should be some sort of rework or buff to her passive because late game it's one of the most pointless passives in the game if she builds critical strike chance. Just my thoughts on her right now.