Shut mesos down gracefully only when the node is shutting down but NOT rebooting.
A clean shutdown will cause the tasks that were scheduled on this node to be rescheduled to another node. Since a rebooting node will return to the cluster shortly, it's better to leave the rebooting node in an unhealthy state so that its tasks continue running on it when it rejoins the cluster.
This has particular importance when you manage your nodes in AWS autoscale groups. When scaling an ASG down you'll find stale agents around.