WF are already at a disadvantage with the new Armor system since they need to spend a feat to enjoy the PRR/MRR benefits from armor...but the Mithral Body feat now is completely obsolete, because it locks you in at 5 MDB forever, which means 5 Dodge.
Mithral Body is supposed to be Light Armor, but actual Light Armor has MDBs that are triple or quadruple that by endgame, while Mithral Body does not scale. Light armor is supposed to be for mobile combatants, classes that have things like Evasion and Dodge bonuses, so it makes no sense that Mithral would impose such a stringent handicap. Right now, there's no reason to take it because (even with some various MDB bonuses) the Dodge penalty is always going to outweigh the PRR and MRR (and AC, meh) you get from it.
Remove the MDB penalty for Mithral Body, or reduce it (5+1/2 Char Level?) so that its a viable option to take.