They always are. If it comes it comes, no sense in worrying about something that (at this time) we can do nothing about. Bruce Willis and Ben Aflac won't be drilling a hole in it to blow it up.
IMO, if there was a big rock comming this way, and the scientists knew for certain that it was gonna hit the earth and was gonna cause major (pardon the term) Earth shaking damage, and there was nothing to avoid it, what would be better? Tell everyone and cause worldwide panic and riots, or let it happen and take it in the nuts? Hard call either way. People are gonna be people and the fact that riots are gonna happen is pretty much a given.
If there was nothing that could be done to circumvent the catastrophy, I'd rather not see it comming. Even still, if it didn't hit, all that panic and rioting would be for nothing and alot of useless damages would have been avoided by not saying anything in the first place.
Look at the moon, all those craters were made by asteriods. The earth gets about the same average, by we got atmosphere that weeds out the small ones. One day a big one will hit the mark, it's only a matter of time.