Asteriod impacts do present a threat, depending on the size of the object. The really big impacts are spread out across very long periods of time, so probability suggest that the very negative issues of Peak Oil, overpopulation, dramatic climate change, ecosystem degredation, societies in conflict over diminishing resources, etc, will be causing major disruptions for the human condition, long before any asteroid comes along to add to the mayhem. Of course the asteroid trackers only find a percentage of the threatening objects, and there is some low percentage of possibility, that a big one could sneak up on the earth, at any time.