People dry hop both ways. The hop ball is more useful when you're dealing with pellets though, and not whole leaf hops. In fact, some of the hop balls won't hold enough whole hops to be very useful in dry hopping.
When dry hopping with pellets, some just dump the hops into the beer and others like to contain them. It's more or less personal preference and dealing with a little trouble when siphoning the beer off after you're done.