I just don't see any situation where I would ever want an Athenes when a Morellonomicon does the same thing, only better.
Once you have Morellos you shouldn't have any mana issues. It gives a ton of CDR, good AP, and the passive is pretty good.
The only thing Athenes has over it is slightly more mana regen, which is unnecessary, and a negligible amount of MR.
Morello doesn't give you enough mana to do extended teamfights without blue buff. Sometimes junglers will keep their blue buff and if you are the main waveclear, you can't rely on morello.
athene gives more mana than morello
morello is better to duel another mage cause of the magic resistance it doesnt look like allot but it is trust me
morello is waay better against things like 
annyone with mad sustain basicly
so if you would want to build somewhat tanky athene
Athene's is gives you slightly better mana regen because of the passive but also gives you some Magic Resist in exchange for some AP. Also, it helps keep mana up in teamfights since it restores 30% of your mana if you get a kill or assist.
