Docker Swarm with etcd
I'm sure someone could give a more specific answer to this question but in my opinion there are basically no advantages of running Swarm with ETCD instead of running Swarm mode. It has been designed with a built-in orchestration to avoid the use of a tier KV store and facilitate the cluster management which is a good thing.
See https://blog.docker.com/2016/06/docker-1-12-built-in-orchestration/
The Docker Overlay driver has existed since Docker Engine 1.9, and an external K/V store was required to manage state for the network. Docker Engine 1.12 integrated the control plane state into Docker Engine so that an external store is no longer required.Source: https://docker-k8s-lab.readthedocs.io/en/latest/docker/docker-etcd.html
One relevant advantage could simply be that you already know how to use it. Docker supports multiple discovery backends in addition to its built-in system, so people who know their way around etcd or consule (which are well established systems) know how to check on information for their swarm setup.
We have this case at our firm and etcd works perfect for us, so we didn't have to work the team into a new technology from scratch.