What practical applications could this have besides self defense? Opening a deck with your fingers results in a cleaner cut than that knife, it seems. And using a razor blade or a box cutter is the preferred method for perfect precision.
I mean, what else would you do with this? Shank your skeptics during a performance? "Oh, the card was on top the whole time, eh?" *stab*
It looks very cool, I'm just not sure why it's being sold by the twins.