I think the only way to make the sandbox meaningful is to *require* it for apps that want to use the other neat features of xdg-apps, which are easier distribution (app bundles) and stable runtimes.

People would prefer to develop against a stable runtime that will work anywhere because it's less work than developing for moving targets of many distributions which change too often. The sandbox would be a "side effect" from them, and it will protect the users from misbehaving apps and some security flaws.

AFAIK, this is already the plan.

-Elad.