They don't know what they're missing.
How about that? I went and tagged a few of my entries because I never can find the useful content hidden in my friends-only posts. (Since those posts don't show up in search results.)So now I've got a spiffy new friends-only tag. As a test, I logged out of LJ and went to my tags page, and the friends-only tag didn't even show up. Interesting! Even the tag itself is scoped to friends-only.
My friends can access the tag, but the general public doesn't even know the tag exists, and wouldn't know about those posts in this blog.