Ghostery does not call itself a tracker-blocker, our users do. This is an obvious oversight for most users, and its somethign that we will address, but at this time, Ghostery is designed to reveal the invisible web and give user the control over it, not make decisions for the users...
As far as the feature, at the implementation time, we've queried a set of users that agreed that when new trackers are added, there is no need to let the user know until s/he encounters it for the first time and reviews it. Obviously, this is another setting we will be moving away from advanced and into wizard so the users may review and select this option at install time.