I’ve made reference to DESTDIR for pkgsrc several times, with only an informal understanding of what it means. From what I’ve learned, and what Joerg Sonnenberger’s told me, DESTDIR support means that packages can be built from pkgsrc without needing to be root. This means local packages can be built on an ordinary user account using pkgsrc.
This also means that pkgsrc can build packages before each upgrade, and only upgrade if a binary package can be built for each item involved. This means minimal downtime and no failures during upgrades, the biggest bugaboo for using pkgsrc that I’ve encountered.