One thing to bear in mind, when it comes to information sharing, is that the problem hasn't always necessarily been one of blocking or fencing off information. Another major problem is simply the massive volume of information to be shared.
I can remember sitting watches where I would receive between 500-1000 messages on a given day... and that was information that had already been sorted down to that which was theoretically useful to that desk. Filtering through all multi-departmental messages and data, to find info that is actually useful, is a lot harder than some may realize.
In many cases, too, the information isn't recognizable as useful until after the fact.
For instance, how many people would have paid attention to twenty Saudis taking flying lessons, prior to 9/11?