It's not a surprise that libspectre was thought to be used mainly (but not only) by evince and okular. I've just committed a patch to evince to use libspectre when it's present, otherwise falling back to gs. I haven't removed the old gs code because libspectre is still too new and it's not yet shipped by distros.

There are many many advantages of using libspectre in evince, some of them:

• One of the most important advantages is probably that it's noticeable faster.
• Postscript printing improvements: print as PDF, multiple copies (bug #443859), reverse and collate are now supported.
• Page labels
• PS specials support in dvi documents (bug #386005)
• Other bugs fixed: #317106, #499787, #501235, #421879, #445797, #486547

A new libspectre release 0.1.0 is now available.

This is the first public release of libspectre

What is libspectre
==================

libspectre is a small library for rendering Postscript documents. It
provides a convenient easy to use API for handling and rendering
Postscript documents.

libspectre is free software and is available to be redistributed
and/or modified under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2, or (at
your option) any later version.

More information about libspectre
=================================

Web site:

    http://libspectre.freedesktop.org/

The programming manual:

    http://libspectre.freedesktop.org/manual

Bug reports and feedback:

    https://bugs.freedesktop.org/ Product: libspectre

The full announcement is available here