You can't buy the half book separately anyway; the bundles come with some books (the already-published ones coming out in paperback or for reissue or whatever) available immediately and others (the ones for which this is first publication) available in a vaguely serial-esque fashion with the first half being available two months in advance and the first three-quarters being available a month in advance.

Personally, I think the half-books function mostly as teasers for the e-arcs. Certainly I've bought more than one e-arc after buying the monthly bundle and reading in advance and being unwilling to stop.