Maybe I'm missing something, but why should we restrict Wikidata to a single standard license (or dual-license)? Why not be like Commons, where content can be uploaded under various free licenses? 

This would have the advantage of letting us respect the licensing schemes of various data sources. For example, much UK govt data is provided under the OGL, while other organisations release data under a ODBL or a CC-BY license.

For user-collated data, we could still encourage or even require a certain license (or dual license).