I don't agree that cc-by-sa is actually "similar enough", certainly not when you include the statements by Creative Commons on the meaning of share alike for visual illustrations.
I feel that the text of the license is far more important then any one particular interpretation of that license. Even interpretations by CC themselves. There are a few aspects that are, from an ideological point of view the most important here:
1) Work is free to use by other people 2) Derivatives can be made, and those derivatives must be available under a free license 3) Authors and contributors receive proper attribution for their work.
Also, the WMF has said that other things are important too:
4) Commercial use must be allowed
If CC-BY-SA covers these important points, that should be what matters most. On these points, I would say that the two licenses are definitely compatible. Of course, there are many other details that are not the same between them.
--Andrew Whitworth