I don't really care about stiffness personally, I prefer a good old flexy steel frame. For the large majority of the bicycle-buying public, stiffness not dreadfully important.

Perhaps Cactus is right, that the advantage of stiffness is very hard to quantify, but you'll have a hard time convincing racers that they have as good a chance at winning a race on a Trek 520 as they would on a Giant TCR Advanced Composite. This has nothing to do with Big-Bicycle Marketing tactics and everthing to do with rider experience.

Take the argument that stiffness is meaningless to the Road Forum and see what they have to say.