Regarding soundstage... In my own experience adding a subwoofer seems to perceptually expand the soundstage, particularly the depth. Larry Greenhill has mentioned the same effect in his subwoofer reviews for Stereophile.
This is exactly my experience.
Further, some of the assertions made in this thread seem to be based on a "perfect world" basis where we have unlimited funds, and unlimited flexibility in our listening environment. I think when you allow for the fact that this is rarely true for most of us, a subwoofer will provide a more optimal imaging than the alternative full range floor standers that may have limited low end due to budget, or sub optimal placement due to room limitations.