Library catalog metadata: Open licensing or public domain?

As reported a few weeks ago, OCLC has recommended that its member libraries adopt the Open Data Commons Attribution license (ODC-BY) when they share their library catalog data online. The recommendation to use an open license like ODC-BY is a positive step forward for OCLC because it helps communicate in advance the rights and responsibilities available to potential users of bibliographic metadata from library catalogs. But the decision by OCLC to recommend the licensing route — as opposed to releasing bibliographic metadata into the public domain — raises concerns that warrants more discussion.
[Thanks Sarah!]