[...] They need all that power to check if a nuke is stale or fresh. Isn't there a boffin with a Geiger counter and a slide rule who could figure that one out? I mean if it's 10 megatons or 9.7 it isn't that important. [...]
It's a bit more complex than that. The "buildz yr ownz nuke!!1!" plans on teh intarwebs give the impression that nuclear weapons are quite simple beasts but today they have evolved in to startlingly complex things; following both the "maximum bang for minimum material (weight)" and "maximum bang for added complexity of <foo>" principles.
An example would be the enhanced yeild tricks fission->fusion->fission types where tritium is injected in to the heart of the fission reaction at /just/ the right moment to yeild a fusion reaction which, in turn, dumps enough neutrons out in to a third stage fission explosive - the whole thing timed just right so that first explosion hasn't reached (and destroyed) that final stage.
The age of the weapon isn't necessarily going to decrease it's yeild (although it might) it may be that it fizzles completely, explodes prematurely and therefore has a low yeild, or (worse of all possible worlds) has it's components mature (decay) in to products which are unstable or start to emit lethal levels of neutrons when brought close to each other.
The boffin with the slide rule would be able to say "yep, there is something in there which is radioactive" but little beyond that.
Personally, unpleasant though this subject is, I'd have preferred if the Reliable Replacement Warhead project had got approval. Dial-a-yeild, safe to handle and long lived.