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.