Unfortunately, this is the kind of debate that can't be conclusively settled except by catching the terrorists red-handed, or by a successful terrorist attack. This from The Advertiser (Australia), with thanks to Jean-Luc:
BRITISH pilots and security experts have accused US intelligence chiefs of "jumping at shadows" as terror alerts grounded more flights yesterday.
British Airways, Air France and Continental Airlines said they had cancelled several trans-Atlantic flights scheduled for yesterday and today, citing security concerns.
Three intelligence officials told the Washington Post the possible threats from al-Qaida included releasing a biological agent such as smallpox or anthrax on a plane so those aboard would spread the infection without knowing it. . . .
Jim McAuslan, leader of the pilots' union Balpa, said he was sceptical whether US security services had discovered a "credible risk".
"We would ask whether terrorists would pick a UK airline," he said.
"UK airline operators have some of the best security in the world."
British security expert Simon Reeve said the intelligence was "tenuous to say the least".
Some within the air industry even suspect the security alerts are commercially inspired. They say US operators benefit from cancelled British flights.
There are suggestions terror suspects are putting flight numbers into communications they believe are being intercepted, simply to cause disruption and anger.
The warnings also come at a time of deep concern about the record of both US and British security services over Saddam Hussein's weapons of mass destruction.
Sceptics also point to the embarrassing false alarm from the US which led to the emergency grounding of six Air France flights over the Christmas period. The "suspicious" passengers at the centre of the alert turned out to include a child, 5, and an elderly Chinese woman.
The FBI claimed their names were "strikingly similar" to those of people wanted for terrorism.
The latest alert is believed to have been prompted by warnings from an informant to the CIA.