Well i had this discussion with a friend of mine about how to build Bard as a support.
Either Ap or Tank i'd give priority to support items and CDR, but the question for me remains partially unsolved...
I stick on the tank build anyway, but first let's speak about the Ap one...
Ap Bard, for me, makes sense in early game, when heals are really really useful, his harraas is much more impactful and you stay more in lane, but here we have a problem... you'll spend your first money to buy sightstone, to close the support item, to get boots, so either Ap or Tank, first you'll spend the money to be a real support.
So a portion of your AP power is lost because you won't get the chance to have AP in early.
Then you have to roam, now if you roam and you are squishy, you can die easily, also only Q and W benefit from AP (and passive too), so i feel AP items wasted on him.
Roaming phase and teamfight phase are close... now in teamfight Bard has his utility, but surely his damage output is not so big (or if you go Mid FULL AP it is, but this is not the case).
SO what Bard does in teamfights? Peels, Ulti, spam Q... slows with autoattacks..
Now if he is AP he dies to easily and cannot peel for his adc or spam his abilities.. also if he gets chased as a tank can survive some autos and some errors can be forgiven, as an AP, no.
The real motivation i think AP bard makes sense is his W, but lategame adcs and fighters would have lifesteal, tanks wuld have warmog.. sooo...
Both Ap or Tank in case i'd buy a Rylai (or a frost Queen eh), but then i prefer the deadman-zz'rot-banner (or similar things) to an AP build, also because i feel the tanky utility splitpusher, seen the mobility is slightly better and more impactful, but still i'm open to the discussion, so if you feel AP bard is better, tell me.