It’s not necessary when the UnFriendly people are humans using muscle-power weaponry. A superhumanly intelligent self-modifying AGI is a rather different proposition, even with only today’s resources available. Given that we have no reason to believe that molecular nanotech isn’t possible, an AI that is even slightly UnFriendly might be a disaster.
Consider the situation where the world finds out that DARPA has finished an AI (for example). Would you expect America to release the source code? Given our track record on issues like evolution and whether American citizens need to arm themselves against the US government, how many people would consider it an abomination and/or a threat to their liberty? What would the self-interested response of every dictator (for example, Kim Jong Il’s successor) with nuclear weapons be? Even a Friendly AI poses a danger until fighting against it is not only useless but obviously useless, and making an AI Friendly is, as has been explained, really freakin’ hard.
I also take issue with the statement that humans have flourished. We spent most of those millions of years being hunter-gatherers. “Nasty, brutish and short” is the phrase that springs to mind.