a couple of thoughts on this:

I really wish I could find a reference of this quote, but isn't there a quote from Sun Tzu's Art of war, to the effect of:
"Sacrificing one of your own army can infact strengthen your forces"?

Of course I don't know what happened on 911.. but there was the theory that it was allowed to happen, to create a rallying point. There was a line in some document in the neo-con philosophy to the effect that the presence of a single, devastating effect was necessary in order to create the sentiment for an invasion of Iraq, IIRC.

Also, there is a theory that we knew of the "surprise" attack on Pearl Harbor, but allowed it to happen in order to create an excuse to enter WWII.

If any of these are true, I have to believe that the decisions were made by expert military strategists, and their concealment was done to simply to prevent public uproar.

There is also some theory that the twin towers were imploded, i.e. they didn't simply melt and collapse due to the planes crashing. Again, even if that was true, it might well have been a life-saving decision overall. Just one that you can't really spell out to the whole population without facing quite a bit of unrest.

just my 2 cents though.