Multiple subwoofers are most useful because, when they're properly located in a room they can help overcome standing wave problems. This is a major benefit and I'm really surprised that no one has mentioned it in this thread. Or maybe I'm just jumping the gun, but after years of ignoring these types of forums I simply couldn't let this pass. If you place the subs according to the information contained in the Harman White paper you'll typically end up with multiple "good" bass locations in the room and you'll also know where not to sit. Especially if you use one of the many room mode calculatiors available on the web.
Follow this link to understanding... http://www.harman.com/wp/pdf/multsubs.pdf