Related to my recent aside about my first "organic" GBS result is this recent and relevant item from Michael Jensen over on Publishing Frontier (the bold emphasis is mine):
I’m still convinced that for small-market publications in particular — the kinds of books that are generally hard to justify significant promotion of — openly indexable content is a precondition for survival, in terms of long-tail backlist success in the scholarly environment. People find something, link to it, and thus promote it for free, for us, in the venues that care about that publication.