A possible source of confusion: do we call "libre compatible" licenses (e.g. CC-BY-SA) "libre"? What about non-libre compatible licenses which do not violate the 4 freedoms (e.g. CC-BY)?
I'm not sure that I buy into the argument that CC-BY is a "non-libre" license.
It doesn't violate or restrict the essential freedoms and I would prefer to label CC-BY as a "libre" license.
Clearly NC and ND are "non-libre" licenses because they violate the essential freedoms.
There is strong opinion on both sides of the SA fence (within the libre friendly community). I don't think that its productive to have the SA versus non-SA teams at loggerheads -- we should respect the core freedoms of choice. I see the CC-BY and CC-BY-SA proponents as two sides of the same "libre" coin. They choose to emphasise freedom from different nuances -- and as an educationalist , I think this is healthy.
I prefer an approach like the Free Cultural Works Definition (not unlike the list of FSF compatible licenses) where we say "the following licenses meet the requirements of the essential freedoms". Incidentally -- Benjamin Mako Hill, who co-author of the Free Cultural Works Definition is a member of the FSF board (I'm sure you know this -- but readers of the list may not). WMF and WE have subscribed to the Free Cultural Works Definition -- the point being that the core freedoms are the foundations for deciding what is libre and what is not. The licenses are arbitrary.
Agreed. Though I think there is a missing nuance among the existing libre licenses.
Benjamin Mako Hill has suggested finding an alternative name for this nuance such as "Libre Ideal License" (Our aim is to produce a definition and license which represent "pure" freedom - another option for those who aspire to such ideals).
So, if we agree on a name for the license, let's say "Libre Puro License" for this discussion, then CC-BY and CC-BY-SA would both be "libre licenses" in the sense that they do not violate the 4 freedoms.
CC-BY-SA would be "Libre Puro compatible" - meaning that "Libre Puro" works could be modified or incorporated into another work and the result licensed as CC-BY-SA because CC-BY-SA perpetuates the freedoms (via SA).
CC-BY would be a "libre license" but not "Libre Puro compatible" since it does not ensure freedom for future derived works (or uses of the work in restrictively-licensed collections, etc.).