Except baggage flies separately from its owners all the time - lost baggage, baggage that doesn't make a connection, etc. The airlines send unaccompanied baggage regularly, bombs or no bombs.
The terrorists that time simply skipped the connecting flight and the bag went on and they didn't. It was also an altitude-triggered bomb which didn't go off on the first flight. It was only when it got on the long-haul segment and got to a certain height that it went off.
After that, anyone who was offloaded, i.e. got sick, missed the flight, etc. had their bags removed before pushback. As a Flight Attendant, that caused a lot of delays but I was quick to point out to angry passengers that it was worth their while. But you're right that if your bag somehow doesn't get on a plane, it will take another flight without you, but you usually find that out when you arrive and not before.
Thanks eepster for correcting me on those creepy 3D scanners. I still understood that sonograms did use some radiation but I never looked into it. I actually specifically requested the minimum sonograms done during my pregnancies, just to be on the safe side.
For those of you who have been through these scanners, how many people (proportionally) were opting for the pat-down? Also, any special treatment when it came to children?