Now, some people think they can compensate for that. But, for me? I don't understand how you could tell that a user learned about me from word of mouth (before they clicked on an adword that got them to my site and a signup.) - To me, figuring that out is so hard that I might as well just focus on the brand-building kind of advertising.
Now, a lot of this is because most people are willing to pay a premium for the 'quantitative' advertising. they are willing to pay extra for things they think they can track. This means that if I'm right (and who knows) then the brand-building advertising, that most people consider largely untrackable, is underpriced.
I mean, I'm not saying I'm right and you are wrong... just that there are a number of businesses that feel this way.