NTP 4 introduces some interesting new things that few people seem to know about, are sparsely documented, and are difficult to setup correctly, however they can help with synchronization in the event of total external network failure (even if you don't have a reference time source).

Now, some reference time sources aren't expensive (others are), but sometimes you care more about node-to-node synchronization than you do about absolute time accuracy. One example might be a large computational cluster where, if the network is disconnected from the Internet for a while, or if the primary time source is down, you don't want the individual nodes to drift apart.