It's not a booster -- I read it wrong. It REMOVES 20% of your bonus MR and Armor and gives it to you as AP. Compare the wording to the booster passives that exist.
That's why the designer keeps saying it lets them keep Soraka from becoming too tanky.
Since AP is very slightly more expensive than MR/armor, it still boosts you a little, but I don't really think, for instance, that Feozen Heart being 80 Armor 20 AP makes it that attractive.
Ouch, I think "soft" tanky items like locket/crucible might be more useful then. I wonder if the Ardent censer might be useful now that soraka has a low cd on her heal.. Probably not, I suppose.