Communities are no hazard, as the maintainer of a community can't read your personal journal's protected entries, because they can't log into communities anymore and anyway I think that got fixed even before the logging into the community thing got fixed. (Well, ok, communities that are run by staff who have the magic priv that lets them override everything for purposes of anti-abuse and debugging, those maintainers could see your entries. But they could see them anyway, and in any case they have to justify it and probably aren't interested in whatever it is you're talking about locked unless you're breaking something or plotting to illegally break something.)
No, this is about alternate personal journals.
This is the journal you started up for poetry, got your friends to add, and then abandoned after the muse ditched you or you started posting it all in your main journal. This is the journal that you started when you were hiding from that creepy stalker. This is the joke journal you started up to impersonate the Vampire Butterfly that Sucks Souls. These are the ten separate journals for each of your ten cats.
These are the journals that your former roommate's erstwhile boyfriend, that scum, never bothered to remove once the relationship was over and he'd been kicked out after his threat to cut the head off your former roommate's little son's favorite teddy bear.
These are the journals that are really a serious security breach for him, because if you remembered that you hold those journals, and you're visiting his journal seeing what he's been up to, and you see those journals still listed in his friends... well, if you remembered the password, or still have that email address kicking around, you could sure see if there are any more posts that you can't see anymore when you're you.
And that's really no good for him.
After things go boom, check your friendslists, don't just remove the major players in the boom. Make sure you're really secure.