Ask HN: How do you feel about optional dependency injection?

Often when writing a library I feel the need to make things simpler for quick and small projects and provide a way to skip verbose dependency injection for non-optional dependencies. I do this by using a non-injected default instance if no other dependency has been provided. I started calling this "optional dependency injection".

I haven't found anything about this approach so far. So I figured I'd ask you guys how you feel about it. Would you consider this good or bad?

Example in PHP:

    public function __construct(Cache $cache = null)
    {
        $this->cache = $cache;

        // Create default cache if none has been provided
        if (null == $this->cache) {
            $this->cache = new SomeCache();
        }
    }