The point is, I prefer to reduce the RNG in my mitigation by improving my change to mitigate than by increasing how much is mitigated. Proc is good for mean mitigation, but point for point, Defence is better for mean mitigation. How many tanks do you see stacking Defence?
IMO the Absorb proc relic is similar to the Sage AoE in PvP. The numbers look great but it doesn't actively keep anyone alive.
upto a point.
if you keep shield static and plot a graph of def% vs Mitigation and absorb% vs mitigation, you will see that specially for an assassin, def stop being better than absorb at somewhere around 27%ish