The main benifit of spikes is to create a stable and solid base for the speaker. No floors and no speaker bases are perfectly flat. By using 3 spikes, this creates a "tripod" type mount that can't rock in any direction.
My guess on hanging the bookshelfs is that the benifit of getting them up to listening height is more benificial than a solid mount. I doubt this would happen with larger speaker/woofer cones. I would like to hear your comparison of hanging vs. a good stand with spikes.