Let's break it down.
LIHOP = Let It Happen On-purpose
Obviously "they" let something happen. Who is "they"? There are lots of different parties. For this post, let me just address the top echelon: Bush, Cheney, and Rice.
They let "it" happen, but we don't really know if they knew what "it" was. Obviously the PDB was pretty darned specific, but it didn't give an exact date and target address. What seems to be the case is that they all went on their merry way and took no specific actions in response to the PDB. If they had known a specific date and place, would they have acted? We'd all like to hope so, but at minimum the American people should have received an answer to just what they did know.
And then there is the really insidious part, which is "on purpose". What would be the motive for that? Well, it is not a conspiracy theory to quote directly from the PNAC document. Clearly these people had already thought through the "opportunity" that would be presented by "some catastrophic and catalyzing eventlike a new Pearl Harbor". And we know that they, in fact, did act quickly on that opportunity. What we don't know is the piece in the middle. If our government was not hoping and waiting patiently for such an event, is there anything that could realistically have been done to have thwarted this.
It seems to me those have always been reasonable questions, and the fact that the commission didn't go anywhere near this leads to suspicions that I consider to be well-founded.
The JFK questions remain open precisely because the Warren Commission obviously swept some important things under the rug. And these 911 questions will remain open for the same reason, like it or not.