add a materialize method to materialize VertexRDD by calling RDD's count

Seems one can not materialize VertexRDD by simply calling count method, which is overridden by VertexRDD. But if you call RDD's count, it could materialize it. 

Is this a feature that designed to get the count without materialize VertexRDD? If so, do you guys think it is necessary to add a materialize method to VertexRDD? 

By the way, does count() is the cheapest way to materialize a RDD? Or it just cost the same resources like other actions? 

Best,