Way back I wrote a piece about the changing model for library catalogues, you can see it here. The main premise was that trying to maintain records in a Library Management System (LMS/ILS) for all the items you want your users to discover is no longer feasible. This is especially true in this here digital age, trying to maintain records for all the e-journals a University has access to is an almost impossible task, and LMS were not designed for thousands of MARC records to be dropped and then re-imported (i.e. sync’d) with another source. And what about all the free stuff, is an e-book not worth being discovered by users because it is free?
So let the LMS be a record of what your library physically holds, and your discovery service a place where users can find (and see how to access) resources that are of interest to their research and work. The former being just one element (albeit a major one) of the latter. Meanwhile your LMS physically holdings can be shared with other discovery systems (such as union catalogues) to show what your library physically contains.