Hence it only being useful 1% of the time.Sure, but you end up spending two turns on it. So if you do a DPR calculation, it kind of drops your sneak attack DPR in half. Doing Booming blade twice will end up doing more damage a lot of time.

I think it's just about the most pointless cantrip there is. Maybe it would have been okay as a 1st level spell and a bonus action so it was a limited resource.