Copyleft/share-alike
Accepting non-share-similarly licenses (i.e. those that do not perpetuate the freedoms in derived works, like CC-BY) would be similar to the GNU Lesser General Public License (LGPL) tactic: - to grow the number of users of libre resources.
Software libraries licensed under the GNU Lesser General Public License (LGPL) may be included with proprietary software.
Typically (and tactically), this is applied where there is already an alternative available to the writers of non-free software.
By allowing writers of non-free software to include LGPL software libraries, we grow awareness and the number of users of free software.
See: Why you shouldn't use the Lesser GPL for your next library.
So far, we have not adopted that tactic for libre knowledge. (The Free Culture movement has, in a sense, via CC-BY)
The CC-BY license is the equivalent of LGPL in this sense. Let's keep Attribution there (:-) and reluctantly tolerate those who don't care for freedom in derived works etc. using CC-BY.
On account of the existence of CC-BY, we do not need yet another nuance of "libre licenses" akin to the GNU Lesser General Public License (LGPL).
Currently, most knowledge and cultural works are easier to mimic than software - e.g. if one wants to replace a proprietary image of a house depicting different compartments/facets of some concept, just sketch an equivalent tree with branches (instead of rooms off a passage in the house).
But it is not always that easy, and may become more difficult as intelligent web services become more common among networked knowledge resources. These are provided via software. Currently we recommend the GNU Affero General Public License for such services.
