The following is a passage from former senior U.S. intelligence official Michael Scheuer's book Imperial Hubris:
Just under the noise, death, and rhetoric yielded by the foregone episodes lies a largely ignored factor that may constitute al Qaeda's main war effort-the steady bleeding of the U.S. economy. In late 2002, Abu-Ubayd al-Qurashi wrote an essay in Al-Ansar called "A Lesson in War" wherein he described al Qaeda's intention to follow Clausewitz's principle of attacking its foes "center of gravity." He said al Qaeda would unrelentingly focus on identifying that point and make "sure to direct all available force against the center of gravity during the great offensive." Al-Qurashi wrote that al Qaeda had studied North Vietnam’s victory over the United States, and found that Hanoi had “fully understood that America’s center of gravity lay in the American people,” and by killing America’s “dearest ones…the war ended with victory on the Vietnamese side.” Al Qaeda took this lesson to heart, Qurashi wrote, but believes that current center of gravity is its economy.
...A conviction has formed among the mujahedin that American public opinion is not the center of gravity in America. The Zionist lobbies, and with them the security agencies, have long been able to bridle all the media that control the formation of public opinion in America. This time it is clearly apparent that the American economy is the American center of gravity. This is what Shaykh Usama Bin Ladin has said quite explicitly. Supporting this penetrating strategic view is that the Disunited States of America are a mixture of nationalities, ethnic groups, and races united only by the "American Dream," or, to put it more correctly, worship of the dollar, which they openly call "the Almighty Dollar." May God be exalted greatly above what they say! Furthermore, the entire American war effort is based on pumping enormous wealth at all times, money being, as has been said, the sinew of war.
Leaving aside jargon about Zionists and conspiracies, al-Qurashi's depiction of al Qaeda's intent seems to mesh with reality. The 11 September attacks, of course, devastated the U.S. economy; it is only now, in early 2004, recovering. But beyond the immediate impact lie massive expenditures-at all levels of American government-that will add permanently to the size and cost of government. In addition to the cost of hiring thousands of federal employees for homeland security purposes; acquiring buildings, equipment, and training to make them effective; and requiring proportionate upgrading at state, municipal, and local level; there lie what must be substantial amounts of unpredictable expenditures for overtime wages-in government and business alike-whenever Washington raises the threat level, or when high levels of security are provided at public places or functions heretofore not seen as serious security risks.
Likewise al Qaeda is at the core of massive increases in defense spending, costs that are likely to accelerate as U.S. officials find the military is not organized, manned, trained, or equipped to fight the kind of wars being waged in Afghanistan and Iraq. Finally, economic planning by government and business must be experiencing significant difficulty in projecting expenditures, given threats of a WMD attack in the United States; the enormous monetary, materiel, and manpower costs of running several wars; the steady diet of shocks thrown into business by steady call-ups of reserve-soldier employees; and-especially in the transport and tourist sectors-by such events as the "emergency" cancellation of flights from Western Europe to the United States in late 2003 and early 2004. Beyond the sound of bombs, then, al Qaeda's attack has continued since 11 September on its notion of the U.W. "center of gravity." Without a second 11 September-like attack, al Qaeda has stimulated immense unanticipated spending, much of which will become fixed in budgets at all levels of government. "Aborting the American economy is not an unattainable dream," al-Qurashi wrote in Al-Ansar. Perhaps he is correct.
It should be noted that even though what Scheuer writes here both was and is true, the fact remains that al Qaeda hasn't done nearly as much to attack America's center of gravity since 9/11 as they could have.
The basic cause of this is that apparently since the 19 hijackers were sent to America to perform the 9/11 attacks, al Qaeda has refrained from sending another team into American soil.
So far all the post-9/11 attacks which could possibly be traced to al Qaeda have either been against non-American targets or have involved single individuals such as the underwear bomber.
While the underwear bomber did quite a good job of provoking an overreaction, the damage the Nigerian did was small potatoes compared to the kind of damage a team of terrorists could've done.
Also there's the issue that individual terrorists are profoundly vulnerable to entrapment by undercover FBI agents, as they so often are moved to seek help from people they met on their own.
In contrast self-contained terrorist teams are nearly invulnerable to undercover agents as they have no need to seek help from anyone but the confederates they met in this or that non-American training camp.
This raises the question: "Why has al Qaeda refrained from launching self-contained team based attacks on America since 9/11?"
I suppose the interesting issue here is whether it's a question of massive tactical failure on al Qaeda's part or rather a rational choice driven by some grand strategic insight which can only be guessed at.