I think the important issue for PG is to create adequately formatted books for the Kindle and Nook.  My most recent contribution to PG was this:

http://www.gutenberg.org/ebooks/39442

This was really a book from hell.  It had tons of family tree tables which I reproduced as ASCII art in the text version and both images and ASCII art in the HTML version.  I used RST as the master format.  The EPUB and Kindle versions of this book do not handle the family tree table (either version) well at all.  HOWEVER, with just two changes to what PG is doing with RST submissions the EPUB and Kindle versions could have been quite adequate.  Those changes are:

1).  Allow pictures which are rotated in the printed book to use the same rotation in the HTML version, instead of requiring it to be rotated "right way up".  Looking a pictures in a Kindle or a Nook you have the same constraints a printed book has, and sometimes you just have to rotate a picture so the width of the picture corresponds to the height of the page (or screen).  If you leave the orientation of the picture alone the Kindle or Nook reader has a better experience and the web page reader has a tolerable experience.  If we consider e-book readers more important that web page viewers (and I think we really should) then we should eliminate the requirement to rotate pictures.

2).  Put some kind of directive in the RST file which says "this content belongs in the text file ONLY".  This would have dealt with my family tree table problem nicely.  Maybe have directives to replace tables wider than three columns with image files in EPUB and Kindle formats, as these readers do not handle large tables well.

I found RST easy to work with and plan to use it on any future submissions I do.

James Simmons