Problem with AP Taric:
Your spell rotation goes like this. Throw DFG + your stun, run in, shatter, ult, run out. Stun will wear off around the time you are shattering/ulting, meaning you are a squishy mage in melee range, with no escape mechanism, you've just lost half your armour buff, and now you're waiting for cooldowns.
I don't think AP Taric's utility outweighs the weakness of having to get into melee range, compared to the many other AP picks. The Dominion aura reduces his heal and armour buff, and other APs have CC that rivals his stun.
I do like AD Taric, picking up Cleaver and Stark's for damage, AS, lifesteal, aura, and the armour shred that stacks with Shatter. His problem is his weakness to kiting, like any melee without a gap-closer, but he can be a surprisingly decent bruiser when you do get the chance to wail on an enemy for a few seconds.