Does Anyone Actually Pick AP Mages Because they are Strong?
In the game's current position, I feel like the only reason people pick an AP mage is to balance out damage types on your team so that no opponent can just build full armor.
Obviously, an AP mage can get fed and hard carry, but I believe that AD champions are superior currently. Rabadons is so expensive that many meta AP champions use items such as Lyandries and Rylais.
Just some thoughts(as someone who mains pretty much only AD champions)

can easily out damage all AD and AD carries on their team as long as the other team isn't 3-4 tanks stacking adaptive helms and spirit visages.