Some of my thoughts on this subject:
I wouldn't mind to get both an ATOM record as a DC record in recordData. I would mind however when I can't count on the availability of a DC recordSchema and an SRU response format.
However, we can't stop people from using an SRU-request for a non-SRU response. Maybe it should even be encouraged to have at least the request being standardised. But the consequence is that SR clients have to know what responses they can expect from SR servers before going to talk to those servers.
Conclusion: we need explain to tell the client what responses it can expect. If an SRU response is not supported by the server it is just not an SRU server. We can't make it mandatory to be an SRU server.
This is not how I would like things to be, but how I see things as they are.
In the meantime we can learn from Opensearch and try to make SRU more attractive.