The Den has Twitter and Facebook sharing buttons for each post, but it uses older API methods that both services will soon be deprecating. Our own universal sharing widget isn't ready for production (though it should be soon) but we don't want to wait any longer to switch The Den to using the One Mozilla theme. Switching to the base theme would mean losing the sharing buttons, which are important to The Den's engagement strategy. So rather than wait any longer to switch The Den to the new theme, we can quickly make a child theme that preserves the old-school buttons but has all the other design/features of One Mozilla. When we get universal sharing integrated into the base theme we'll no longer need this child theme, but this is a stopgap in the meantime.
Agree on the not making the theme be blocked by the universal sharing widget. You can follow along in the universal sharing progress in bug 723761 and all of its dependent bugs.
The child theme went to production this morning (r105623). The next step is getting IT to enable it in WordPress, then we can complete the switch.