I would have a lot of use for this as well. We want to link Planner to Sharepoint List and updates would be done depending on the bucket location of tasks.
This, so much! We use a Planner board to track the status of long running activities. It would be so helpful to know when something moves and be able to trigger the board observers then...
I suspect they are not considering it because it could potentially create a LOT of triggers in a short period of time. People happen to move around card freely a lot. In certain cases.