I think a lot of people show pride in their work by finishing out the trunk along with the rest of the car, since that is an area that usually rusts first. I for one kept mine closed because it was pretty ugly, but now that I'm getting the car completely redone, I will be opening it up .
I agree, a nice trunk seems to dictate who thorogh the car has been detailed, the first thing that people usually skip out on is the trunk, then under the hood. By showing a nicely detailed trunk and engine bay, it just proves to people there is nothing hiding. Lots of cars look great with the trunk and hood down.