Dmitry Gutov wrote:
> Why don't we want to use straight apostrophes in the source files again

‘again’?  Emacs has been using asymmetric quotes in doc strings forever.  Or are 
you referring to the original Bug#20385 proposal of straight apostrophes? 
Stefan wasn't sure he liked that idea, and preferred curved quotes.  I'd be 
happy either way, myself; the point is that those grave accents have got to go.

> Or keep the current quoting, and likewise prettify it with font-lock?

That's been proposed, but nobody has taken the time to do it.  Plus, it'd be One 
More Thing; it's simpler if doc strings are plain strings and are not little 
subsets of Markdown or whatever.

> this looks bad in Thunderbird's attachment preview

It looks OK when I use Thunderbird 31.6.0 in Ubuntu 15.04 in an American English 
locale (first attachment).  It's not a thing of beauty, admittedly, but it's 
legible enough.  It looks much nicer under emacs -Q, which is what counts 
(second attachment).