I think discoverability is one of the main advantages here. Opening citations results in making available additional paths to reach the citing and cited publications, where these paths become accessible to anyone. In addition, if you don’t make this information available to the public, all your journals and publications cannot be used by applications, analyses and derivative databases that use these open citation data. As a compendium of this argument, at OpenCitations we often use an analogy to introduce the advantages of this open availability of citation data at a big scale.
About the effort needed (and without knowing your particular situation): if you, as a publisher, are already depositing these citation data to Crossref, then you only need to ask them to turn on the reference distribution for all the DOI prefixes you control. However, if you need to extend your current publication workflow so as to produce these metadata to submit to Crossref, then the things can get a bit more complex. The task could be easily automated if you use a machine-readable format for storing the sources of your publications (e.g. XML), otherwise more ad-hoc solutions are needed, of course. I guess you are in this latter situation, aren’t you?
