"a comparatively costless and therefore puzzlingly rare decision"
It's important to have a single interface for administering the zone, which all DNS servers slave (replicate) from; otherwise you risk inconsistent results from different servers.
Unfortunately, many of the registrars don't allow you to mix their own DNS with third-party secondaries which slave from it, or don't allow their own DNS servers to slave from some other primary.
Popular example: Amazon route53 does not allow additional secondaries (AXFR), nor can it act as secondary itself
Relatively few providers support this. Godaddy is one example which does: if you manage your zones with them as primary, you to add additional secondaries which AXFR from Godaddy (you could just run a £10/month cloud VM for this); and equally you can make your own nameserver the primary and Godaddy will slave from that.
Even if you find two providers which support this, I think it's fair to say that most people don't understand DNS well enough to set this up; they therefore rely on a single cloud-hosted provider to manage it on their behalf. Sad but true.
Biting the hand that feeds IT
