The "stay in your seat during the last hour" rule isn't completely frivolous. Bombers arriving in the US are likely to wait until they are over their destination city because it will make the bombing more dramatic. The goal is fear, not deaths. Also, people tend to procrastinate suicide bombings.

Obviously, stewardesses telling people to stay in their seats won't prevent bombers from detonating their bombs, but it may make their behavior more noticeable to other passengers.

I'm very surprised that there still isn't some screening machine that detects all explosives. How hard can it be? It'd be much nicer to just walk through a big sniffing machine than all the other ineffective rigmarole.