Yet, none seems to be able to point to an incident where it mattered.
I read of one. It was a case in which a woman whose husband had often left a handgun loaded with a lighter-recoiling handload (.38 Special) out for her to have access to. The night came when she attempted suicide. The method was not declared in the account I read. He locked the gun, and all his others, away, and she got treatment.
A few years later, she had seemed "back to normal" for quite some time, and expressed another desire to have a gun available for her defense when she was home alone. Her husband, according to the account, again put out the revolver, loaded with a light handload, for her availability.
He came home one night to find her with the gun, according to his account, in a suicidal position with it, and tried to get it from her. It fired, and she was killed.
He was charged with her death, and the ammunition remaining in the gun was cited as evidence. Because some of it was loaded in cases marked "+P", that was the factory ammunition load it was ballistically compared to in the trial proceedings. His pleas for investigators and the prosecutor to examine the rest of his own handload stash fell on deaf ears; it was, after all, evidence "manufactured by the defendant."
I cannot remember the sentence, but he was indeed convicted of at least a manslaughter charge.
Part of what hung him up was the fact that the ammunition was "fabricated" by the defendant, presumably (for the prosecution) for the purpose of assuring his wife's death. I'm not sure if the outcome of the trial would have been different had the ammunition been factory-sourced. I don't see where it should have been, but the opportunity for the prosecutor to portray the defendant as having fashioned, in a sinister light, the death rounds himself, would have at least been eliminated.