We'd like to keep the licenses for contributed data very clear for users. Should we require contributions to be one of CC licenses or PD?
Multiple layers of datasources ordered by license
It is clear multiple datasource exist that offer very high resolution support under a certain license. This license can be for example CC-BY-SA-NC, there is no a priori reason to not aggregate this datasource in the website if the user of the site decides he can obey the rules of this license. By specifically offering a CC-BY, CC-BY-SA and for example NC layer the community is served in a better way.