atma is good because of scaling ad and possible critical hit.. really prefect item for garen and sion... ;\
And that's why Riot nerfed, and then removed Atma, as well as made Tytanic Hydra give damage scaling with health instead of flat AD. By giving flat AD, Atma increased not only the damage of auto attacks, but also abilities that scaled of AD, so the more health somebody had, the more their abilities dealt damage. You heard of Atmog? Or Metagolems? That's basically what became of Atma and Riot decided that it can't exist in such form. And when they introduced Tytanic Hydra, which works similarly like Atma. they just didn't repeat the same mistake and made only Cleave and Crescent damage scale with total health of the user.