From the potential forces that they see in crashes I can see where the double halo comes in. Not only do they crash but they also get full impact hits after they crash. Crazy stuff.
Some of the engineering white papers that Nascar has done on their chassis' has shown that the area of the Cowl/A piller is one of the key structural areas in maintaining chassis rigidity in the cars with their speeds and over the duration of their races. Improvements in this area translate directly to improved suspension performance and driver feedback.
Plus, like Vince said, the added safety of this layout with the types of impacts they sustain is never a bad thing.
that downtube has so much bend in it I would think it would fold like paper in a roll-over situation, especially at the speeds they tend to roll them at. It doesn't surprise me at all they saw a lot of chassis flex due to that. Note where the triangulation is added, it is all to support that wicked bend, and then the downtube from the top center also adds similar rigidity by bracing the center of the halo. I remember the first time I saw how bent those tubes were in the cars from inside camera shots - what a horrible idea.