I'm one of the developers working on the WorldCat API. My take is that
the API is evolving and adapting as we learn more about how it's
expected to be used. We haven't precluded the addition of more record
metadata to OpenSearch responses; we opted not to implement it until we
had more evidence of need.
As you've noted, WorldCat API OpenSearch responses are currently limited
to title and author information plus a formatted bibliographic citation,
while more complete record metadata is available in DC or MARC XML in
SRU responses. Until now we had not seen a strong push from the API
early implementers for more record metadata in OpenSearch responses,
based on direct feedback and actual use. I can see how it could be a
useful addition, though, so we'll look into it.