> Wouldn't it be safer all around if the subparsers took different arguments, or at least different namespace 'dest', than the main parser?

IMHO, yes.  I agree that the semantics of what the original code is trying to do is quite ambiguous.  Since the documentation says that parents= causes all the given parsers and adds their options to the parser being constructed, I'd take that to mean that the -v before the command, and the command's -v would point to the same dest.  The admonition for add_help=False seems to reinforce that.

The fact that this used to work should be considered an accident.  I wouldn't call it a regression because the documentation does not make it clear that this should work.  I think this is not a bug.

Feel free to reopen this if you disagree and and cite a convincing argument for sharing dests.  To be totally unambiguous, use different destinations.

FWIW, I've never used parents myself.  I've always done something like what honcho eventually landed.