I chose Zookeeper to achieve this same goal a while ago (before I heard of etcd). I have been pleasantly surprised at how useful having a coordination service is in my infrastructure in addition to a configuration management service. Because of this, even though etcd looks like it serves distributed configuration management better (aka simpler), I'm happy with my choice of Zookeeper.

Two examples of how a coordination service has been useful:

* cluster wide throttles to help protect overwhelmable backends

* redundancy in maintenance cronjobs that really only want to be run once per cluster per time period

(edited for formatting)