Posting my Highlight again cause i would like to receive constructive criticism and possibly someone to play with who also mains hanzo I rarely use the scatter arrow unless i panic or need to take down a tank quickly
You should use scatter to just take out an immediate threat. It's not that long of a cool down and it's almost always a kill. Always try to hit the supports in the back when you're on high ground but don't let the dps rain damage on your team either. Create pressure and make them have to try to kill you but stay safe and run to tanks. Don't be far from your team.
Why would you not use scatter? You can win games with that alone when the other player cries about your lack of skill in chat instead of try and kill you!