User blog comment:TheTobarMethod/Do AP's Need A QSS Upgrade?/@comment-4888212-20130903115934

i dont think casters need that.

AD/autoattackers oftentimes need to constantly attack to deal lots of damage, therefore, a stun hurts them alot.

whereas casters can drop their entire combo (maybe not chars like cassio), and retreat, resulting in a safer playstyle. stuns on APCs are only used to burst them, which they have zhonyas for.