Ruunan's is only really good on Teemo and Kalista.
Teemo can spread more poisons and his short range means he won't have as big of a positional disadvantage from using it. His speed steriod also means the AS won't go to waste.
Kalista actually gains mobility from AS so Ruunan's is perfect, none of the AS goes to waste. It also allows her to build up rend stacks on muliple targets and since she hops around, the positional disadvantage from using it isn't so bad.
On any other ADC, it forces you to have bad positioning to be the most effective and the AS is actually a waste. The reason Phantom Dancer and Static Shiv have Movement Speed on them is that, you can have a 2.5 AS, but if you are chasing someone, or kiting, you have to move before you can shoot. If you only get AS from an item, you end up wasing the stat, because your Auto Attacks are limited by the time it takes to move, not your AA speed.