The number one reason I recommend a runner on stairs is safety!  The traction gained from a carpet runner on stairs saves pets, kids, and adults from slipping.  I usually prefer a stair runner that has texture.  Patterns and colors are a bonus, but the texture not only gives more traction, but shows less wear.  All in all, most textures and patterns are a bit more forgiving with traffic paths and spills than a smoother solid weave.

Cat on Black and White stair runner
Placing a stair runner down the center while exposing 3″-5″ on each side is the most attractive way to gain the traction!
Photo credits: image1: nelson hancock photography for image 2: