Why would anyone need that when there's ghostery and noscript? Furthermore, what benefits does that feature have over ghostery? Seems like ghostery blocks more than that feature does, which makes it seem like that's a waste of effort.
I suppose it might have some use if it's actually on by default, but they haven't stated that it will be, and if it isn't on by default, then anyone who would know to turn it out would be able to install ghostery anyway.
The benefit is that this is built in to the browser, meaning more users will use it. If you want the extra protection of Ghostery, then by all means, go for it. This is easier for other users.
Well, for one thing, Ghostery is veeeeery slow (or was last time I checked). For a second thing, what makes you think that it's going to block less than Ghostery?
It allows you to block things coming from known tracking domains, giving better privacy and faster page loads on many sites.