Users find it a waste of resources to deploy 3 big PostgreSQL instances just for HA where 2 suffice even if they deploy 3 lightweight DCS instances. Having only some of the nodes act as DCS and others purely PostgreSQL nodes will reduce waste of resources.
A big problem is that putting your DCS into Postgres means your whole system is now super-sensitive to IO/WAL-streaming issues, and a busy database doing database stuff is going to start affecting the DCS stuff. With three lightweight DCS servers, you don't really need to worry about how stressed the database servers are. In that way, I feel etcd et al. are adhering to the unix philosophy of "do one thing, and do it well."
