what is the importance of subject?

i've been thinking about this for a while. i bring it before you now, so that you can add your questions to my own, or if i'm lucky, so you can lend me some hard earned wisdom. put generally, i'm wondering what exact role the subject plays in a photograph.
specifically: if art can be used to make a statement, if portraiture is a window into the soul and if fine art is said to have meaning, then how are these meanings changed depending on what the lens captures. for example: two people take two different pictures. ansel adams sees beauty in the shape and form of sand dunes. warhol uses a picture of marilyn monroe for his art prints. these are drastically different forms of photography. next, take the guy who takes pictures of birds with a ten billion times zoom lens. take war photography. is there any merit in calling any of these 'better' or 'worse'? or are they all just different categories of art, which happen to also all use cameras? what do you think?
'but wait!' you say. what about when it's not two different forms of art? what about two people who go about trying to say the same thing with a photograph, but they use different subjects to accomplish the task? i want to know what social critique a photo of a flower could possibly provide. or a bird.
i suppose part of what i'm asking (and i might also be asserting it) is whether there is a point where beauty becomes secondary to the purpose of the artist, a mere tool in the box alongside shock, fear, line, texture, color, you get the idea. a picture of a flower or a bird is nice, and if it's pretty, great! but what role do these types of subjects play in art, a term which should be preserved for something higher than mere beauty.
too often i bring my camera into the forrest in order to take a picture of the forrest, because i want to see something beautiful. so when it comes down to it, i suppose my question is this: how can we begin to see the variety of unused images available for visual artists to use to make informed statements about life with, and in what situations are those images more valid than others?