Right now you can bind Elasticsearch to only a single address with either:

network.bind_host: "some_ip"

or

network.host: "some_ip"

It'd be convenient to be able to bind to multiple hosts like so:

network.bind_host: [ "some_ip", "some_other_ip" ]

In particular it'd allow the convenient configuration:

network.bind_host: [ "some_data_center_accessible_ip", "localhost" ]
network.publish_host: "some_data_center_accessible_ip"

Which would allow elasticsearch to respond to requests to localhost.