A lot of people have made some very good points, so I'm not sure if what I'm gonna say has already been said. Either way, here goes! My dad has always had this saying and it goes like this: a guard dog is only as effective as the person behind him. In a nut shell, a guard dog can bark and bite all he wants; if there is no person (preferably with a gun) to back him up, he will cease to do his job.
I think that this applies to roosters as well. Yes, they're amazing animals that will live and die for their flock, but they have their limits. I like to think of the rooster as the first line of defense NOT the last. Provided you get a good boy, he will always be on guard, warn the flock of predators, escort his hens to safety, and jump in-between his girls and certain death, but if he doesn't have a human to back him up, the whole flock is toast. Think of him like a burglar alarm. He'll give warning and do his best, but he's not meant to be the last defense.
With that said, I think roosters are pretty awesome! My own little guy is a big jerk to me and my sisters, but he is absolutely the BEST little man with the hens. He gives all of his treats away (seriously, we have a hard time giving him anything); calls hens over for every morsel he finds; escorts hens to and from the nestbox during the day; breaks up fights and even gives little pecks to the perpetrators; warns everyone to take cover when he sees hawks, while remaining outside himself; literally screamed to one of his hens who was being swooped by a hawk and jumped out as bait; and I've even seen him grab mouthfuls of food and give it to the lower hens, who have a hard time getting at plates of leftovers/treats. I mean, ya gotta respect that! He's also pretty easy to manage. He will flog and bite if you give him the chance, but I just pick him up and give him face scritches and that's usually the end of it.
So, all in all, I think I will always have a rooster around. They're generally way more protective of the flock than a dominant hen would be, and you also get the added benefit of being able to hatch eggs. If you manage to get a really nice one, then you'll probably never go without one again!