What's the point of having both action='store_true' and default=False in parser.add_argument?

There's no point. From the docs:

'store_true' and 'store_false' - These are special cases of 'store_const' used for storing the values True and False respectively. In addition, they create default values of False and True respectively.

(added bold)

In the comments, Charles Duffy said, "It's a stylistic choice to be explicit rather than implicit", which is a fair point, but it also means that if you're editing the code and accidentally mismatch the action and default, it'll break:

>>> parser.add_argument('--name', action='store_true', default=True)
>>> parser.parse_args(['--name'])  # Good
Namespace(name=True)
>>> parser.parse_args([])  # Bad
Namespace(name=True)

And I think the implicit default is obvious anyway.

The __init__ for the store_true subclass is:

class _StoreTrueAction(_StoreConstAction):

    def __init__(self,
                 option_strings,
                 dest,
                 default=False,
                 required=False,
                 help=None):
        super(_StoreTrueAction, self).__init__(
            option_strings=option_strings,
            dest=dest,
            const=True,
            default=default,
            required=required,
            help=help)

Notice that it sets default=False. The user code can override that, but what's the point? This subclass is just a store_const where the default is False and the const is True.

add_argument takes a number of keyword parameters and creates an Action subclass object. Different actions make use of different combinations of parameters. add_argument takes a casual approach to required or superfluous parameters. That is, there isn't a lot of code that checks that just the right parameters have been defined.

I'd leave it off since the default is the correct one.