Well, it looks cool, but according to the article it needs to be flying above the missile sites when they're launched. So for it to shoot down missiles, we'd 1) have to know where they're coming from and 2) have a civilian airliner circling that location nonstop waiting for the launch. Those sound to me like pretty severe shortcomings. If we knew where missiles were coming from and could safely overfly the area, it seems like we could just bomb/cruise missile the site before launch.
It would be used during something like Desert Storm or Iraqi Freedom where we know that enemy ballistic missile launches are possible and where they will most likely originate.
Actually, it could be quite a ways from the missile launch site. One of the "here's how it works" graphics I've seen had it orbiting about 100 miles off North Korea. Simple enough to keep one there and protect it with fighters from a carrier group of the Wolfpack F-16 guys in South Korea.
Civilian airliner circling non-stop reminds me of the P3 that was forced down in China not so long ago. Maybe they could vaporize a fighter or two for practice.