If I got 1,000 visits per month to an ebook site through content marketing, I could only count on about 2% joining my email list, and around 9% of those ever buying (3% across three pitch emails). Multiplying those, content marketing would only drive 1.8 sales per month. Clearly some people are finding much more success than that - which of these numbers is off?
I have landing-page content marketing opt-ins that routinely perform from 30 to 40% (http://cl.ly/image/1i3z1K3z1K3b) and one of my clients has a site-wide opt-in converting from 6 to 10% of site visitors.
The key to numbers like that are a strong match between your site traffic's intent and the opt-in offer. As an example, any time I guest on a podcast, my answer to "how can people find out more about you?" is to send them to http://positioningcrashcourse.com (which is not about me at all, but is a premium content resource that serves as the mouth of my funnel). That's the page that converts as well as 40% and normally converts at 30 to 35%.
I call this "getting your own traffic", and it converts at very high numbers relative to paid traffic or even organic search traffic.
So if you approach traffic generation in this way (or another way that sends warmed up, qualified traffic to a landing page), your 2% traffic --> list number is potentially way off.
The other reason why you're seeing so many others talk about content marketing as the holy grail is because it _is_ the holy grail... of building trust and demonstrating expertise.
It is not the best way to drive short-term sales tho.
But if you have a more medium to long term view, content marketing is a great way to attract leads and nurture them towards being ready to buy.
If you're an author (who is by definition sloughing off content all the time), this is a viable strategy because you're in it for the long haul with your audience, and time is on your side (see "Your First 10,000 Copies" for more details). If you're trying to drive e-book sales for minimal cost, it probably is not a great strategy.