"What every set of stairs need."
Or, "The best dressed stairs are wearing them."
I'm such a carpet snob, I guess that comes with my job! But I always think stairways look better with a runner. It makes me sad to see a beautiful set of stairs left bare.
You can go neutral or bold.
Stair runners are a great place to make a "statement" with a lot of color or pattern. Since it's a small area that you're moving through, they don't seem to get too busy.