Preferential trade and economic efficiency

Daniel Altman makes (what I believe is) a novel claim:

Already, we’ve seen rich and poor countries shifting their attention to regional trade deals. Diverse groups of countries can do a lot by trading amongst themselves, exploiting differences in costs, resources, and technologies. Pretty soon, we should see a few large regional blocs dominating global trade. The ones that lower trade barriers faster will grow faster, too. In these cases, the poorer countries would be expected to catch up to the richer ones. When that happens, the wealthier blocs will start to look at the blocs that lagged behind for new trading relationships, and the barriers will start to fall between the blocs. In the end, we’ll have something very close to a global trade deal – and we will have arrived at that deal in a much more organic, economically efficient way. [emphasis added]

Yes, preferential trade is an importance force shaping the WTO talks. Yes, preferential trade is more politically palatable than multilateralism. But more economically efficient?

Most who are relatively optimistic about the dynamics of preferential trade don’t claim it’s the best path (Baldwin: “(1) Regionalism is here to stay; world trade is regulated by a motley assortment of unilateral, bilateral and multilateral trade agreements; (2) this motley assortment is not the best way to organise world trade”). How would large trade blocs be more economically efficient than a global trade deal?