For a while I was for the idea, but I'm now against it.

Neph put it excellently.

If person X can't view certain parts, all he has to do is join and poof, he has the info anyway.

This may make people want to join just to find out what's in these posts. Or it could make people not join because they want to see what they're about to join, but can't so they leave annoyed, searching for another forum...

Hmm..