On one hand, then guests can't really see a part of a thread that might be a good way to gauge what kind of community we are. And in the case of a staff member (or ex-staff for that matter) making their posts invisible to guests, rules threads aren't viewable and possibly other major stickies or events that guests might be viewing.
But on the other hand, I have had an ex stalk me on here before just to join and berate me. And I feel like it wouldn't have happened if he hadn't have looked at all my posts and such. I don't think that happens too often, but still.
idk. I'm torn. :( Maybe instead like...a feature that hides the username and other parts of postbit of the person instead? That way a guest isn't missing out on what's being said, but doesn't exactly know who it is.