Why would osama waste his time with light aircraft - Heck, take 20 litres of petrol to the local night club and burn, baby burn...
That doesn't carry nearly the effect as using an airplane. The goal isn't to kill or to maim, or even to damage property. Terrorism. That's what it's about.
You haven't seen it happen yet, but you probably will. As I said before, following 09/11, a third of the flight schools in the US went out of business. I was personally assigned to retrieve a heart the evening of the day it happened, and I had to watch the individual who was to receive the heart on the television, crying and telling the public and the media that he would die because the aircraft that was to go get his replacement heart wasn't allowed to fly. That was me, the one who was supposed to make that flight. One of thousands upon thousands of flights that didn't go.
For years after that time, I had to put locks around my propeller every time I walked away from the airplane, keep at least three disabling devices attached at all times, add devices to prevent the aircraft from flying, received frequent visits from the federal bureau of investigation, and had to obtain additional clearances and undergo additional scrutiny. Those of you who come from overseas to do training have some inkling of the hoops through which you must jump...all because a few individuals obtained a few airplanes and did damage.
Today, it doesn't matter how much damage is done. I'm a long time ag pilot, and fire pilot, among other things. In the years following 09/11, the public hysteria over "white dust" being dispensed by ag airplanes lead to multiple groundings of ag aircraft, prompting some to sell their operations and others to go out of business. We were prevented from flying on fires at times, as a result. No ag airplane was ever used for a terrorist act, but the effects remained. Had an ag airplane ever been used, one could safely discount agricultural aviation in the US. Over. Kaput. Done.
Same for several light airplanes being used. The amount of physical damage to be done? Minimal. The amount of psychological and economic damage? Inestimable. Massive, and potentially without end. Blow up a night club...that happens from time to time. Nobody closes all the night clubs. Blow up a truck, nobody is going to park all the trucks. Blow up a ship, there are plenty more ships. Blow up another airplane, and much of the aviation industry may not be able to recover from the fallout. In the general aviation sector this is especially true.
How many of you are instrument rated? Most private pilots are not. Never the less, the matter of requiring all light airplanes to fly under IFR was seriously debated following 09/11. Much as I love VFR flight and personal freedom, I'm not entirely against that, then or now. It's not entirely workable, but the concept was certainly discussed that anything deviating from an IFR plan would be considered a threat, and has been on many occasions in the US in some areas.
Even today, a light airplane violating the airspace around the nation's capitol in Washington DC is enough to stop the country's political machine, evacuate congress, and send enough firepower to vaporize a large city to intercept it. That's the kind of response even a light airplane gets today. So, you ask, why would a terrorist consider using a light airplane? Do you really have to ask that?
When Mattias Rust landed at the Kremlin with his Cessna 172, nobody in the Politburo was laughing. Nobody thought a Cessna 150 could have landed (crashed) on the Whitehouse lawn. But it happened. You might not take it seriously, but rest assured that the public does, and the body politic does, too. So long as that's the fuel of the legislation that decides your privilege to fly, you should take it just as seriously.
The lost life and damage done is irrelevant and really quite meaningless. The loss of freedom and the public panic that follows in it's wake is what a terrorist act is all about. Take it seriously.