Trivial things like plot holes don't make a film bad
Signs is an all together very well made film. From directing and acting to cinematography and music, its all top noch. And these are the things movies will be remembered for, not whether or not a certain plot point made sense. A piece of art should not be deconstructed in a thousand pieces, rather it should be examined as an entire creation.
Its by no means a perfect film but it archieves what it sets out to do. Its highly engrossing, thrilling and has an emotional punch. Its shot perfectly and has a fantastic atmosphere. Signs highlights great aesthetics and I can only praise it for that.