Riot** HEAVILY** favors AD, don't expect the bias to change, hasn't changed since season 2. They nerf AP in a manner that seems reasonable but they flat out ignore the brokenness of AD. I mean seriously, most of the champions people rag on about hating are all AD, you seldom hear much about AP champions.
Its because of this bias that support healing has been nerfed in the ground constantly alongside spell vamp, and yet they straight up ignore the clearly busted nonsense of having 20% or more lifesteal and healing for 50-100 damage** OFF EVERY AUTO EVERY SECOND**; yeah no problem here... meanwhile spell vamp gets removed for "brokenness" and support healing nerfed into the ground even though it has a good 8 second cooldown atleast for most of it...** NOT EVERY AUTO EVERY SECOND, NO THATs BALANCED**!