I saw 'Signs' recently, and didn't really even think about the absence of guns. I mean, I guess it's one of those things where the premise of the movie isn't 'realistic,' so why should the reactions be? I think it would have taken something away from the movie, surprisingly. They (sort of) explained why the aliens weren't using weapons -- the book they were reading claimed that the aliens would come via hand to hand combat to prevent the use of nuclear weapons by the earthlings.

Regarding believability, I think that's worthy of a separate post.

Concerning the aliens non-armament, I understood that the deterrant against nuclear warfare was the fact that they were interspersed with the population. Giving them a few good zap guns would have helped their odds while not changing this.