Before SERVER-14118, both _id and host were mandatory fields in the configuration document passed to rs.add():

http://docs.mongodb.org/manual/reference/method/rs.add/#pass-a-member-configuration-document-to-rs-add

As of commit 2e68421 which originated as pull request #688, _id will be computed using the same method when passing the host string to rs.add():

http://docs.mongodb.org/manual/reference/method/rs.add/#pass-a-hostname-string-to-rs-add