Forgot twitch, dude's base AS is really high, but not quite as high as kennen's.
Pretty much every good AD carry has an AS steroid
From this, I predict that AD kennen is bad
Well, ADCs that don't have AS steriods:
Ashe, Cait, corki, vayne.
Also, you gotta consider varus has less AS when the minion version is active, and only half a dagger more AS when he killed a champ.
edit cause ninja'd:
I'll give you Cait and Urgot, but Ashe and Corki have AD-scaling skills on low cooldowns (eventually low, in Ashe's case) to make up for it. And Sivir has her ult.
That more is their general playstyle, neither ashe's volley nor corki's rockets really replace an AA, The former gives kiting potential, the latter gives poke and burst damage, it should be considered versus kennen's other advantages, not just dismissed as "basically equivilent to an AA steriod", that's silly.
Edit 2, and urgot is an AD caster/anti-carry anyway. He just lanes in the ADC slot.