What, I wonder, is the chance of a passenger airliner being shot down by such missiles...? To what extent does this - small - chance counter the also small risk

What, I wonder, is the chance of a passenger airliner being shot down by such missiles...? To what extent does this - small - chance counter the also small risk of airplanes being used by the terrorists? I agree with C. Stross that this is a daft idea.