When you type an option that requires an argument, but don't provide one, you get an error message that says there is a missing argument. There should brobably be a hint on what values are acceptable. For instance, there is no way to know from "guix package -v" or "guix package --help" what values are accepted. In other cases, there is a --list-something option.
Maybe -v is the only case where it would be useful to do something, because you could guess its existence and not use a verbosity level. Maybe fixing the help message to say --verbosity=[0-5] is enough?