Usually one of the girls will end up being "the boss"- whoever is at the top of the pecking order, most likely. Some particularly dominant hens even go so far as crowing and other roo behaviors in the absence of a roo, but that's not common. Usually one hen will be the leader, but might not be as dedicated/involved as a good roo would be.
Girls have their own hierarchy, roo or not. I can't see anything but good coming of getting rid of a nasty roo. I no longer have a roo and my flock is much more seemingly content and I know I am for sure. As for protection from predators or at least acting as sentry, I'm not convinced- my flock uses squirrels as a sentry and they whoop the butt of any roo (where I live, at least).
And possibly most importantly, I don't get any earache from my wife about crowing!