If you think of curriculum as a work-flow/sequencing tool, in the old 3x Curriculum there was no branching, just one order to the links. The success criteria was viewing the url. Simple and useful. The Polyxena Curriculum document describes a system with one order to the links and success criteria of passing a test.
Other use cases include, pre-tests, reviews if the user fails the test, i.e. branched sequencing. Also some lessons might be passed without tests (just viewing the material), some only by instructor approval, and maybe some with some other criteria, like posting to a bboard. Thus we need the ability to specify different success criteria.
I am totally in favor of implementing only a subset of the general case functionality, with the subset being driven by the needs of whoever is doing the work. But it would be good to put references to what is not being implemented into the design documents as well as hooks and notes to allow the next developer to add on easily.
We put a little of our vision on how we saw Survey moving into the future in our RFP. I uploaded it into the .LRN extranet on SloanSpace. Email me if you need access and don't have an account.