I use Kopr-sheild on cut threads otherwise nothing. It won't stop leaks into the conduit as conduit couplings seem to be(are) running thread(no taper) so they won't seal anyway.
I can think of a reason for steel over PVC if transistioning out of a deep hole. If compaction of the the dirt isn't done right the PVC will shear off where it hits native soil. I've seen it many times around tank holes. even rigid will shear if their's a coupling at the transistion
