I think it'll be a temporary thing. Look at Bethesda, for example - they experimented with crap DLC like horse armor, but then quickly turned to solid, fairly priced mini-expansions.

Age of Empires Online had to overhaul its pricing. Microsoft Flight crashed and burned (har har) after trying to sell new planes and areas bit by bit.

If Zynga does fail in a serious way, it should hasten the demise of the "social gaming" business model.