For me, it's about MY time.
Having dined both downstairs (and on Friday for lunch), and upstairs, I like the reservations, and seating upstairs, BUT I have done both. I find that I have less, and less time, to stand in line, and the same goes for my guests.
If one has never dined downstairs, then the line is a must, to experience the tableau. Hazelhurst makes some great points, and offers a useful tip, about times/dates.
I also agree on the dress, regardless of a stated, enforced "dress code," but that is probably just us.