There will never be enough sky marshalls for every flihgt. Every flight has at least one pilot, usually two. At least one of them should be armed and if neither is, the passengers should be informed of this when they buy their tickets. See if people want to fly with armed or unarmed pilots. :)
Not sure I agree! The bad guys should be thinking that every flight has at least three and as many as five (pilot, copilot, flight engineer, and two plain clothes sky marshals) armed good guys. Reluctantly, I must say that arming the flight attendants is not desirable as they might be more easily disarmed.