It would be a good idea to extend the same privacy protections to custom lists as to the watchlist and favorites on the profile.
I think people should have the option of having a profile but also having the option to control which lists are hidden from public view, including watchlist and favorites.
I plan on looking into more specific privacy controls in the future. Some things like Favorites are heavily integrated into the site (profile, used for recommendations, etc) which makes it tough to allow that to be private. Watchlist might be possible though.