I'm not sure why mental stuff is harder to defend against. From what I understand it's because it bypasses Speed and Toughness powers and Athletics, etc defenses, is that right?
But if you can come up with a justification, like, "Well, he can only direct his attacks at a visible target, so I'll dodge with Athletics to make it harder for him to track my location, or hide with Stealth, throw up a Glamour, etc." It should reduce the difficulty of defending. Okay, maybe a brick type character would still run into problems since the Toughness powers only affect the Physical side of defending, but there are the Homebrew Mental Toughness powers which should help. Plus, I think someone mentioned giving those at a discount if other Toughness powers are possessed.
I'm not arguing for or against by the way, just trying to wrap my head around the problem right now.