Earth has been impacted by asteroids in the past, so there's nothing to worry about. It's just a natural phenomenon. Besides, the people saying we should be looking for asteroids are just greedy for grant money. If it turns out the be a real threat, I'm sure the technology to deal with it will magically appear -- with the economy the way it is we can't afford nonessential projects now.
Remember how silly these arguments sound when applied to other potential problems.