Things like this is exactly why I believe people should carry.
Flash mob violence is one of the very LEAST likely scenarios you'd face where you may need a gun to save your life--even in a big city. Flash mob victims, when compared to "ordinary" violent crime victims, wouldn't make a pimple on a water buffalo's butt.
Half of America lives in towns not big enough to support flash mobs. And yet they still have crime victims as a result of the same ol' criminals who've been victimizing folks like you and me for ever.
Not saying that mob violence isn't something you shouldn't consider or be prepared for. Just saying that it's not the BEST reason for carrying a gun.