I think the idea is that while the revelations that Zerbster & co receive are of course the true word of God, the outside world isn't prepared for it. Like they are old bottles and they don't have the freedom of spirit that we lucky (whoops, luck is from the Devil, as MO taught us
) people who live in TFI have.
So they (normal Christians) would choke on the radical doctrines and the devil (or the horde of new demons) would make them stir up trouble for us, God's endtime brides.
And then of course the government would do something about it.
So we'd have a lot of persecution, the goverment would ban us since the devil will influence them, etc, so it's really just less trouble if we keep hold of them. You can view them once the Lord touches your heart and lets you see the emptiness of the system.
Also we are trying to keep them secret from the antichrist, so he won't know about all the secret powers we're developing.
Seriously though, I think it's because they know that many Xers turn against them, so it would be dangerous for TFI if their secret pubs were easily accessible to Xers, as they (the pubs) would certainly be capable of being put to use against them (TFI).