I work for a library and scholarly publisher looking to improve reference metadata. I was curious if Crossref, or other members, would recommend trying to make reference metadata as complete as possible, comprising sources that lack DOIs – or conversely whether people think references could simply be restricted to a handful of DOIs (and leave the other DOI-less citations to be found in the paper itself).
If I’m interpreting your docs and general ethos correctly, the primary purpose of reference metadata is DOI-to-DOI, so one wouldn’t necessarily need to include either free or structured text on sources that lack DOIs. But I could be off base, would be curious to hear others’ thoughts.
We do encourage the inclusion of all references, not just those that are currently registered with DOIs.
It’s true that providing reference counts and cited-by information is one of the ways we use references. But references, in their own right, are valuable to end-users of our metadata even if we can’t match them to other registered DOIs.
Additionally, just because an item doesn’t have a DOI registered for it right now doesn’t mean it won’t have one in the future. Archival and backfile content does get registered. And, we do periodically retry stored references that were unable to be matched to a cited work’s DOI at the time of the metadata deposit to see if they can be matched at a later point.