php - Exactly how does the defaultValue work in Symfony 2.0 NodeDefinition?

I'm trying to expose the semantic configuration within a bundle in Symfony 2.0 but I'm having trouble getting the defaultValue to work in the NodeDefinition class. I generated an empty bundle and created the necessary files for the Configuration to work and I am able to get the config value, but I want to set a defaultValue to configuration item. I use the defaultValue() method and remove the configuration item from my config.yml and then it shows up an empty array? Can anyone explain how the defaultValue() actually works, am I missing something?

<?php
// ./DependencyInjection/Configuration.php

namespace Test\Bundle\TestBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

class Configuration implements ConfigurationInterface
{
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('test_bundle');

        $rootNode
            ->children()
                ->scalarNode('foo')->defaultValue('bar')->end()
            ->end();

        return $treeBuilder;
    }
}
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-

<?php
// ./DependencyInjection/TestBundleExtension.php

namespace Test\Bundle\TestBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\Config\FileLocator;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\DependencyInjection\Loader;

class TestBundleExtension extends Extension
{
    public function load(array $configs, ContainerBuilder $container)
    {
        $configuration = new Configuration();
        $config = $this->processConfiguration($configuration, $configs);

        var_dump($configs); // empty array

        $loader = new Loader\XmlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
        $loader->load('services.xml');
    }
}
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So from the above Configuration class surely when the config item 'test_bundle.foo' is missing, it's value will be set to 'bar'... yes? Well that's what I thought but it doesn't.