When creating invoices via Xero UI, contact and organization default due date rules are applied, which is great!
But when creating invoices via API, due date field is left blank unless it's explicitly set in the API request. All this nice logic of applying contact and organization due date, in that order, is skipped for API created invoices, resulting in a rather inconsistent UX for end users.

And to "use" that feature of Xero, each API application would need to reimplement same logic that Xero UI already has.

I think keeping parity between UI and API would be very good to do here, so Xero users that rely on that feature for manually entered invoices can enjoy the same benefits when their invoices are imported via API, by any add-ons they use.