"I do believe - hard to think of anything scarier than reports of airliners disappearing over the Atlantic, 1 by 1 by 2 by 1... "
I disagree. It's just a matter of opinion. Planes exploding above cities would be worse. More death. Not just people on the plane. They LOVE murdering innocent people.
Not really more death on the ground. A plan crashed in NYC after 9/11 and I don't believe there hardly any deaths on the ground at all.
Perhaps the infiltrator that they had in their midst misunderstood their intentions...
By the time of arrival there wouldn't be much fuel left on board; the explosive would be limited pretty much to just what was in the bottles. A plane coming apart over a US city wouldn't appreciably add to the death toll unless it just happened to plunge into a stadium full of baseball fans, and this is unlikely given variables such as timing and the terrorists not having control of the cockpit.
Taking down 9 planes or so and all the passengers and crew would be spectacular enough for terrorist tastes. The side benefit of not giving the US another "ground zero" to rally around- just a generic wet spot in the Atlantic- is enough for their tastes- they know the long drawn-out recovery process of workers after the 9/11 attacks riled America up instead of making us quiver in fear as they had hoped. [Note that recently Palestinian propagandists were quick to stage their own "ground zero flag photo op" in Qana in the Lebanon conflict, and so on.] The economic impact on us again, is enough for their tastes. Perhaps they could in the hopes of further dividing us, exploit conspiracy theorists, sowing rumors that the aircraft were taken down by the US Navy, etc.
On the other hand, the terrorists would miss out on powerful imagery for recruitment since there was not likely to be any photographers in the mid Atlantic to take dramatic photos of their dirty deeds.