For FallenWings, I really wasn't attempting to say that it was required, just pointing out one of the issues with Dark Knights, which is how difficult they are to optimize. Being split between two damage types in this game is extremely painful,. Also, DoTs aren't effected by "elemental" resistance down, for example Storm's Eye, however they are effected by things like Trick Attack, the Autoturrets, and Foe Requium. Also, yeah... magical DoT on a physical attack.
Edit: Shurrikhan and I posted around the same time, but yes, Salted Earth is physical. Because of this, Salted Earth, outside of something like the Balance or Battle Litany, is impossible to boost. Most physical DoTs have this issue though.