- WorldCat Discovery
The value you can get from turning on these databases is the citation search results. You won't get full text links with them, but with the citation you can still enter an ILL request or buy the content for your users. That can be useful for filling in weak content areas you don't collect in. It is also useful inside the A-Z Link Resolver for identifying where a journal is indexed (ie. when a Reference librarian wants to know where to search to get citations for a journal).
The drawback is that Index-only or Abstract databases add a lot of additional results to your searches that aren't immediately available. If you set them to Treat as Held in the Service Configuration > Metasearch Content > Default Databases and Licensed Content > Configure Datatabases to search tab, WorldCat Discovery will show these citations as Held and move them to the top of your results. For that reason, many libraries enable these databases for searching, but don't set them to Treat as Held.