Twisted Fate has always been the guy you could play AD or AP. One day people were playing him AD and then for some reason people just started playing him AP. If Kennen can get a "Secondary: Marksman" tag can you put one on TF Riot? If not he might just be patched buffed/nerfed into a pure mage one day with no hopes of hybriding at all.
AD TF is still viable, and I run it with success regularly. That 2 sec stun every 4 secs end game lets him 1v1 most any champion. His lane presence in bot is actually very strong while he is cycling his PAC, and is great for last hitting and sustaining. I've found he runs bot lane exceptionally well with any support that has a stun to accompany his, Nami being my personal favorite.
I'll agree that he has a high learning curve, and he's strengths are subtle, not to say that they are not potent. That just makes it all the more rewarding when you stomp an Ezreal and Sona comp.
