Theme layout files often contain multiple redundant sections, e.g. a header section that's identical across 1, 2, and 3-column layout files. fred wrote a simple include()-esque renderer for theme_easy (see: https://github.com/FMCorz/moodle-theme_easy/blob/master/renderers.php ) that allows themes to easily break things up into their component parts, easing development.

It'd be nice to see this be something we could count upon in core.