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In my opinion, maybe SF, Unix.SE and SU could profit from Docs.SO quite well.
I suspect it could also be useful on at least some of the non IT-related sites.
From another point of view, there are some extra features which exist only on SO (jobs, +2 new review queues, etc). So maybe Docs.SO is planned as an SO-only extra feature, too.
It is also possible, that the core communities of some sites simply won't need such an extension.