A timer is much more difficult to implement and requires more parts than using the home button (for example). Doable? yes, but it's much easier to spot etc. And again, yes in the future they might implement a timer, but this might derail a plan that is already in motion, causing noise etc. in the terrorist community.
In all seriousness, I think it would depend on the quality of the intelligence they have. Certainly if one cancels a flight just to board another flight that goes through another airport there will be a red flag (I would hope so, at least). Only the most idiot of terrorists will carry on a plan that has been publicly busted by DHS.
In theory, it might not be "safer", but if we look at real-world history we can count many planes that crashed/exploded due to human intervention, many more that were at high-risk due to human intervention (including the shoe bomber), and more that were targeted but the plan failed. I think that there was only one crash because of batteries, and it was a cargo carrying thousands of batteries.
So while I agree that storing a bunch of batteries ain't the sharpest idea in the world - at least in theory - IF the intelligence is good enough it might be a good way to patch a much more pressing and urgent issue. Sometimes you can't just kick the can down the road.
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And considering that the US already has all the info about all the travelers that planned to come to the US, he/she will be flagged the moment he/she tries to board the next plane.