Great post. I think this is worthy of a steady conversation, metadata always seems to be shifting in terms of application in the eBook world.
I have the same questions about keyword/search term metadata. Do online retailers actually recognize this field? I feel like it would be the "webby" way to present BISAC data, but it's unclear if it's worth the effort or not. Obviously, "discoverability" is the goal here, and it seems that custom keywords could lend a much more specific representation of the content than an archaic categorization system.
I'd love to hear any/everyone else's experience + expertise on this one!!!