A good rooster is great protection for the hens, even going so far as to put himself between any predator and the hens. If they are free-ranging a good rooster will not only lead the hens to tasty tidbits to eat, he'll allow them to eat first and offer them bugs he's caught from his beak. Then there's the whole fertilized egg thing, making it handy if you ever want to try your hand at incubating without having to buy fertilized eggs.
Disadvantages include the fact that roosters develop spurs that can injure someone if he decides that person is a threat to his flock or if he just turns out mean. The rooster may decide to challenge you for top spot in the pecking order. Some people simply cannot handle the mating ritual that goes on between a hen and a roo. It's not pretty and a hen can lose feathers over it or be injured.
I will never be without at least one rooster. My chickens free-range and I can't be out with them non-stop, but my roosters are and they do their part to keep my flock safe.