OK, I am not arguing that it doesn't exist, just that it isn't a very significant organisation. Security experts can debate on its changing strategy as well.
It just seemed odd to me that an organisation which is allegedly as strong and powerful as Al Qaeda is supposed to be launching the 9/11 attacks would have quit when its enemy was on its knees (or down on the ground). Despite what people will say, 9/11 was a victory for the terrorists since the east coast of the united States shut down for the day and US air traffic was pretty much shut down for a week. It seems that an organisation with such strong skills at organising such a task would have planned to keep kicking while the giant was down. Instead, the attacks pretty much stopped.
There was the Beltway Sniper attacks a little over a year later that paralysed the US Capital area, but that was just a loner. We can also argue that the Fort Hood Shooter was "Al Qaeda inspired" instead of just another crazy with a gun. Not to mention at least two conspiracies to shoot up military bases in the US, Fort Dix and Quantico, that have been thwarted so far.
My point, however,is that the ability to terrorise large segments of the US population using a firearm is pretty well known, yet no one has been successful. This is an avenue for terrorism that has remained wide open despite an alleged "war on terrorism". Additionally, is the amount of gun carnage and the fear it creates also terrorism? I know that citing to VPC for a link between guns and terrorism won't persuade the unpersuadable. In fact, a few more DC sniper incidents wouldn't persuade them, but that's irrelevant since the point is that guns are tools and they are tools that terrorists can use for their ends.
Also, I have to admit it was odd that no one took credit for 9/11. Most terrorists will take credit for their actions. It seems as if the 9/11 attacks were attributed to Bin Ladin and Al Qaeda as a reason for all sorts of other actions which make no sense in a "War on Terrorism". Terrorism can be a homegrown act committed by anyone (e.g., Oklahoma City) and is not unique to a "Middle Eastern-looking man with a bomb."
The Oklahoma City bombing was the most significant act of terrorism on American soil until the September 11 attacks in 2001, claiming the lives of 168 victims and injuring more than 680. The blast destroyed or damaged 324 buildings within a sixteen–block radius, destroyed or burned 86 cars, and shattered glass in 258 nearby buildings. The bomb was estimated to have caused at least $652 million worth of damage. This was an act of home grown terrorism perpetrated by some one who was upset at the federal government's handling of the Waco Siege and the Ruby Ridge incident (Oddly enough, people of this mindset scream for "gun rights" to fight government tyranny).
It's easy to look for the bogeyman outside of your country, but harder to deal with internal threats to national security.
Anyway, my point is that whatever threat "Al Qaeda" is to the United States is minimal. The US's illogical reactions to 9/11 prove that to be the case. The problem is that these reactions have utilised US (and other countries' resources) in activities that have not been productive. That's fortunate since Al Qaeda would be having a field day if it were truly powerful.