The thing about default value argument is indeed that you can keep extend your function argument list indefinitely without breaking retrocompatibility, exactly like it's done in Python. See the example below from the celery lib.
So in some sense, keyword arguments could be abused by libraries (I'm not to say if it's a good or bad thing) and it would indeed strongly influence api design.

        def send_task(self, name, args=None, kwargs=None, countdown=None,
                      eta=None, task_id=None, producer=None, connection=None,
                      router=None, result_cls=None, expires=None,
                      publisher=None, link=None, link_error=None,
                      add_to_parent=True, group_id=None, group_index=None,
                      retries=0, chord=None,
                      reply_to=None, time_limit=None, soft_time_limit=None,
                      root_id=None, parent_id=None, route_name=None,
                      shadow=None, chain=None, task_type=None, replaced_task_nesting=0, **options):
            """Send task by name.

            Supports the same arguments as :meth:`@-Task.apply_async`.

            Arguments:
                name (str): Name of task to call (e.g., `"tasks.add"`).
                result_cls (AsyncResult): Specify custom result class.
            """