I like
Psionic Powers much. I use it for my campaign and I introduce abilities in it to new players, for simplicity's sake, like this:
- "Psychic Armor is a type of mental defense. It works against any damaging psionic attacks."
- "Psionic Shield is another type. It gives a bonus to resistance rolls to non-damaging telepathic threats."
To those players, declaring "Too bad, this enemy's attack ignores your DR from your Psychic Armor, because he's bought 'Mental Defense Only' enhancement" will sound unreasonable. I, as a GM, know that there's game mechanics behind the scenes -- Psychic Armor ability is actually a modified Damage Resistance advantage and Mental Defense Only attack is unstoppable by DR -- but my players don't always know the mechanics.
PU4 discusses general enhancement and doesn't mention to special enhancement. Malediction-Proof is one of the special enhancements for DR, so I wanted to know the designer's intention about whether Malediction-Proof works or not. If the designer supposes Mental Defense Only to be a sort of conditional Cosmic Irresistible, that's okay. I'd reserve it for a new enemy with "new threat", as the_matrix_walker suggested.