Why is it in core?

The replication solution is actually two separate non-replication features that are useful on their own, but when used together, create a solid replication solution.

PostgreSQL has been able to load point in time recovery (PITR) log files from one server onto another, creating a warm standby solution for fail-over. Recently, 8.5 gained the ability to serve read-only queries while performing recovery. This feature is called Hot Standby. In a separate feature, Streaming Replication, PostgreSQL will be able to send PITR data across a TCP stream in addition to the log files. Combining Hot Standby with Streaming Replication provides a solid master-slave replication solution, suitable for fail-over and load balancing of read-only queries.

This replication solution happens to be in core because that is the right place for Hot Standby and Streaming Replication to be. It is also a very low overhead design because it is based on log files that are created during normal operation anyway.