User blog comment:KetakuShai/Riven Rework/@comment-10428734-20141105183923/@comment-8506165-20141106081842

"I could list a billion mages with at least .5 AP scaling on their shield, and you can get about twice as much AP as you can get AD."

...usually with higher cooldowns, further limits on their usage thanks to not being manaless, and no synergy with sustain from items because spell vamp is worthless on 90% of mages.

It's not quite as one-sided as "mage shields are better so Riven's is fine".