A derivative work vendor could distribute only printed copies of the manual, this would make editing it harder. I don't see why it wouldn't be ok to allow providing a source via an URL in the manual, so it wouldn't make physical distribution more difficult than when using licenses without source requirement.
I like the idea of a transparent copy being the source: there is some free software with source that can be edited or compiled only using nonfree software, FDL solves this problem for manuals using it.
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