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I assume they have some deterent value.

You can tell because some of the failed bombings (like the shoe bomber) failed because their plans were stupid to get around security, and if security wasn't there they would probably have used a normal bomb and succeeded

I have no idea if it has worked or not but you got to count deterrence too. If you have a lock and alarm in your house it might deter someone from even trying to break in. Of course you could never know if the deterrence worked (only attempts would be noticeable)

I don't think that the question is really "removing all checks". It's rather "are all those expensive machines necessary?".

I mean, they do find a ton of guns and ammunition. I wouldn't be so sure.