This is kind of what I was thinking about when I asked if it was a good or a bad idea. I wondered if teaching a dog to bark AT a person might be stirring up trouble. After all, I think they will bark anyway if they sense they need to.This doesn't require a full regimen of protection work, but does require some suspicion work and basic agitation work by a competent individual. While it can be handy, really think about if you need it and the potential disadvantages it can bring in certain situations... waking up suspicion in a dog who was previously social but now has been rewarded for the opposite and now you have to manage, potentially rewarding fearful reactions as Lies mentioned.
And your point about the alert bark needing suspicion/agitation is enough to make me say, not for us. I don't want to agitate my *family dog* who, as Diane wisely said, needs to be friendly with kids.