Alan Schwarz in the New York Times writes about David Neft's "on base advantage", a measurement where a walk is valued by subtracting a player's slugging percentage from 1.
However jarring to those riding the modern walk bandwagon, Neft's refinement makes perfect sense. From the pitcher's standpoint, a batter expected to slug 1.000, on average, should always be walked because his average hit is more damaging than a walk.
Meanwhile, walking a player with a .000 slugging percentage is grounds for an early shower, because he is no threat in the first place. The higher the slugging percentage, the less costly the walk.
Walking a player with a .000 slugging percentage is sometimes referred to by those of us on the West Coast as "The Grabowski Principle." You can get something of an explanation of it here.