I've never seen a middle-lane hogger responsible for anything dangerous either - I'd put them firmly in the "annoying" category. Once in a while you see someone responding dangerously to someone driving in an annoying but otherwise basically safe way, and I also see this when the "annoying" way people are driving is driving at the speed limit, or waiting for a safe gap to pull into after overtaking, so I don't feel like that can really be blamed on people hogging the middle lane.