Setting a default for an option doesn't really seem to do much (besides showing the default value in the help screen:

$arguments->addOption( array('name', 'n'), 
    array(
        'description' => 'Set the name",
        'default' => 'Foo Bar'
    )
));

Doing this:

echo $arguments['name']

returns nothing, but it should return "Foo Bar" unless the default has been overridden.