If I combine your thoughts with the DM option to (perhaps FREQUENTLY) disallow the power, or allow NPC #32 to ignore it, then it becomes workable. My issue is just primarily that I am opposed to 'zapping' PCs powers at the risk of making something they've 'spent' a power slot on less valuable than a different choice.
The better option for me, if I were ever to run 4E again, would be to simply remove CAGI, but I'm not sure if that 'nerfs' fighters entirely or not.
Shrug... The game talks about using common sense at times. it's your game do whatever makes the most fun for yourself and your group, only you know what that is, not me or the rules or the designers of the rules.
Here are some example of how I see CAGI playing out:
1. Average Warrior vrs a Foe
PC: (I use CAGI) "Maglor lowers his blade slightly shifting his body as if he has slipped..." (Roll roll roll A Hit!)
DM: The Goblin smirks seeing the opportunity. It steps forward blade drawn hoping to end this fight perhaps sooner then he'd planend...
PC: "Maglor smiles as the goblin takes the bait... Straightening back into fighting for he swings his weapon into the unsuspecting goblin..."
Or
Roll roll roll A Miss!)
The goblin smirks seeing the opportunity. "I'm wise to your trickses human! But thank you for lowering your defenses!"
2. Poor farm boy vrs trained warrior
PC (I use CAGI) Johan holds the sword in his hand the tip swaying from side to side his body shaking in terror... (Roll roll roll A hit!)
DM: The warrior smirks knowing he has nothing to fear from such a small boy... Stepping forward he says "You're brave boy, but unfortunately for you bravery will not keep you alive much..." Suddenly you see the warrior wince, as he slips on some blood falling against the blade in Johans hands...
or (Roll roll roll a Miss!)
DM: The warrior smirks knowing he has nothing to fear from such a small boy. Stepping forward he says: "You're brave boy, but unfortunately for you bravery will not keep you alive Much longer..."
PC: Terror grips Johans heart... He's no match for this warrior...
3. Crazy Kung Fu/Jedi Style
(I use CAGI) Wo-Pan smirks, lowering his blade and motioning for his enemy to come to him... (Roll roll rolll a Hit!)
DM: The goblin tries to resist, but is no match for your warrior magic... he steps forward at your call feeling your blade slash into his body...
or (Roll roll roll a Miss!)
DM: The goblin smiles... "Foolish warrior,... did you think I was a petty commoner open to your trickery??? But I will take this opportunity you've given me..."