>> Anyway, I think we can agree that setting `line-spacing' to nil is 
>> *not* analogous setting \baselineskip=0pt in TeX.
>
> I'm TeX-illiterate.  Can you explain why these two aren't analogous? It 
> sounds like they are, when I read the definition of what 
> \baselineskip=0pt does.
>

\baselineskip=0pt means that the height and depth of a text line is equal 
to the maximal height and depth of the characters in that line.  See the 
attached two screenshots that demonstrate the difference.