"It may be that you are not yourself luminous, but you are a conductor of light."
-- Sherlock Holmes, The Hound of the Baskervilles, Ch. 1
August Derleth named the hero of his Sherlock Holmes pastiches -- the character who is almost but not quite Holmes -- Solar Pons. The name, appropriately, is Latin for "bridge of light."
Dr. Watson, on the other hand, is a "conductor of light." In the context of the quote, Holmes means that Watson's mistakes in his attempts to draw inferences from Dr. Mortimer's penang lawyer set Holmes off in the right direction. This is a valuable service, to be sure, but not the only way in which the good doctor is a conductor of light.
Most importantly, if it were not for Dr. Watson's accounts of their cases, so frequently dismissed as sensationalistic by the Great Detective, we most likely wouldn't know about Holmes. Watson leads us to that bridge of light.
Watson also led Holmes in some good directions -- away from drugs, toward some good cases (the engineer's thumb and Col. Warburton's madness), and into those all-too-infrequent periods of rest and recuperation from the investigative labors that often tried Holmes physically as well as mentally.
For all of these reasons, I don't think I've ever met a Sherlockian or Holmesian who couldn't be called a Watsonian as well.