Agreed that Protection is generally less brittle than high Defense, ethereality, etc. The one thing you have to especially watch out for with Protection, though, is fatigue, since even a 30 Protection can take significant damage on a lucky critical hit at fatigue 45 or so.
I've found that Wraith Lords benefit from good Defense because 1.) they already have high natural Prot and Ethereality, 2.) beating good Defense requires swarm tactics, 3.) swarms are just more fodder for Soul Vortex.
-Max