It's a shame AppC is going away[0]. OCI is pretty much just a formalization of what Docker already does, so we're stuck with (IMO) implementation details like layers as part of the core image spec.
While layers are codified into the OCI specification, the manifest system is flexible and allows for future extension. The main goal of OCI is to provide a specification for what is already widely deployed without introducing too many new concepts.
[0]: https://github.com/appc/spec/blob/master/spec/aci.md#image-m...
Disclosure: OCI Maintainer, Docker Employee