I'm not replying to your post because I genuinely believe that that 9/11 was an inside job. However because you asked such an open question I would like to give you a concievable reason best explained in this short explanation of the "GOD" business. A little annoying to point you toward a film (on my part I would rather have solid referencing material). But it's hardly as if I could quote a higher up of American politics as being the one who said it without spouting bull****. That said, maybe someone will cut out the middle-man and start quoting the film as an authority figure on the matter xD
http://www.youtube.com/watch?v=ZUuBQya7XTYEither way, instability is the greatest factor in the ability to make money (a stable market is not one with much capitalist oppertunity of buying low, selling high). And the best way to create instability is to wage war. However this would be typically unacceptable to the masses as justifiable logic. So you need to create a desire for the masses to want the war you do. As mentioned in the clip, the three big things are Gun, Oil and Drugs. Now, does war help the gun trade? Incredibly. Have the Americans waged war in one of the richest natural resources of oil? Yes. And the other factor which I am more able to comment on, is that the wars in Iraq and Afghanistan have opened countless new supply lines for heroin. Infact there have been countless scandals that barely got the attention of the public of British soldiers bringing Opium back into the UK by the ton, Ofcourse this was blamed on the "low pay" soldiers recieve. And it is directly responsible (in reality, difficult to prove on paper, but you can see a correlation between the war starting and heroin availability in the UK and America).
And needless to say in recent years the acceptance of drugs as a "social" activity, or even a pass-time for some is noteable in the UK. The benefit for this with governments is that even though it is money made illegally, which they rarely get to see. A large number of drug dealers are materialistic which means the money they make on the black-market becomes legitimate income which benefits the system. The taxes on the cigarettes and alcohol they buy, the rent they pay, Their cars which chew through petrol like kids through smarties. So long as a dealer isn't violent it is rare that the police force will pick a bone with them, and although I do not believe this to be an actual belief of the police force. A system has been built in place that leaves them no choice but to overlook these (and other) crimes, due to manipulated factors of prison availability, CJS, workload/over-reward. And no incentive to tackle what keeps them employed.
Anyway I never intended for this post to be this long haha, mainly just wanted to give you a "rational" reason that a country would attack it's own population under false pretense. Even if (and I can't stress this enough) it is not something I believe myself.
Truthfully I always thought that if it was an inside job, it would be for the benefit of covert warfare. (I'll explain briefly).
After attack on your major city which you planned, and the world's eyes on you. You blame an otherwise non-existant organisation and give it a figure head, labelling them as your enemy. But what happens is, this non-existant organisation starts to exist through the war you started in "retaliation". So you now have "extremists" in another country, wanting to blow themselves up for any reason that could "harm" your country. These extremists are blind and narrow minded, brain-washed even. Which you can manipulate very easily (simply by having spies find and recruit them under the pretense of "death to whatever you're against, in this case America and everyone else") and use them in circumstances where using your own military would violate numerous treaties and ethics, and risk war.
A little under-handed maybe, but when it comes to relationships between countries that don't make the paper. There is a lot of underhand business, and even more from countries that are always in the eye of the global public xD.
I like to think my theory is proven after they killed "Bin-laden" because before the first party popper was even let off, they started talking about "expect retaliation". Which just gives me this inkling that we can pretty much count on retaliation when it benefits the US most. But we'll see, I always hope I'm wrong when my brain thinks like this. =P