Good points, everyone! However, like Grant, "I'll fight it out on this line if it takes all summer." I do see the logic of "care for your horse, saddle, gun self" and the point about older western being made with original, well worn guns is quite insightful.
Perhaps it's a question of aesthetics; super shiny brass, to me, seems quite out of place on Civil War-era guns in a film set almost twenty years later. I have brass doorknobs that turn dull yellow just a few weeks after cleaning. Any vets who've had to polish brass belt buckles and rank insignia can attest to how quickly brass fixtures, especially ones that are handled frequently (like revolver grip frames and rifle receivers), tarnish up. If this weren't true Brasso would be long out of business.
So Hollywood, listen up! Before you turn out another western, send your guns to me for free weathering, tarnishing and break in. After all, I just want to do my part