1. Would you really take a chance with that five year old and just let him continue to make random calls? What if he was calling random people in the local phone book?
2. Contrary to popular belief, our broadcasts aren't getting out that far. They fade into background noise after a certain point. A tight beam intentional broadcast on the other hand, can go quite far.
3. "Needlessly hostile" implies that it wouldn't be "needlessly easy" for some civilization that is thousands of years ahead of us to exterminate us. Those are not good odds.
4. Why take the chance? Better to wait as long as we can before announcing ourselves to the neighbors. Make sure we are secure in our capabilities first.
There are a lot more reasons why it's a bad idea than a good one.