Rich, your correspondent fails to note that what he calls a “canard” has an important flipside, which explains why – in the context of a trial – it’s not a canard at all.
If the state fails to prove that the shells were consistent with the gun (i.e., that the weapon could have fired the shells), the clever defense lawyer – who only needs to sow some doubt, not to prove what really happened – pounces on this omission. He argues to the jury in summation: “How can you possibly find that the prosecution has proved its case beyond a reasonable doubt when we don’t even know whether this gun could have fired these shells?” This can be a very powerful argument for acquittal, so the prosecution has to nail it down if it can.
Look at it a different way. Let’s suppose the district attorney had decided, as your correspondent suggests, that the consistency of the shells with the gun was so inconsequential it wasn’t worth proving. If the jury had convicted Williams anyway, what do you figure Mike Farrell and the rest of Hollywood would be saying now? Call me a cynic, but I have a feeling they would not be saying the “probative value” of that piece of information was “almost zero.”