You can do many things to spark curiosity in visitors to a landing page. Some may grant you a few extra minutes to follow their curiosity... but then what? Most of them will get an answer to their questions and then stop using the service. We see this on checkout flows where people will put something in their cart because they are simply curious about shipping costs and taxes. Then the site owners wonder why there is such high cart abandonment.
(I'm a co-founder at Sidelines.) LanceJones, you're exactly right: A key lesson from our landing page experiment is that vague messaging incites curiosity in users, and curiosity can be a powerful motivator to get users to spend an extra few minutes to sign up for a service. This same user may have decided to bounce off the landing page if it went into the details of the value proposition (this is what we did with the initial versions of Sidelines' landing page), so the vague messaging has a better chance of getting the user through the door. However, whether or not the site is able to keep the user engaged post-signup depends on a number of different factors: The engagement numbers for sidelines have consistently gone up as we keep optimizing our sign up flow once the user logs in, the first run experience of the product, the welcome email, email/FB notifications, newsletters, etc.