At 2 items it works out to on avg less than 1 damage per auto, in exchange for 5 AD.
The loss of 5 AD means that even if your spells have no AD ratio but you have Kraken Slayer, it's already a net loss because of just the AD ratio on Kraken Slayer. Factoring spells into it just makes the calculus even worse.
Even if you didn't go Kraken Slayer, taking Caitlyn as an example, you lose 24 damage from Q+R+W on a single target, so you'll need to auto ~24 times to break even at two items. I didn't even factor in headshot's reduced crit multiplier here.
edit: A few more examples: Draven loses out at 2 items and maybe 3 items because Q bonus AD doesn't crit so he loses damage per AA on average. Ashe loses out at 2 and maybe 3 items because Frost Shot, loses out even more if she goes Kraken. Jinx will go Kraken. Twitch is roughly even at 3 assuming no Kraken. Only Samira is in most situations a net gain even at 2 items.