I'm not sure if this card really needs another review, but as I've progressed in the game I've realized finesse moves make this card really invaluable. Additionally this card isn't restricted, so it can be piled up with itself and other defensive attachments.
As I've understood action windows better, I find shifting this card around can be key. Siege phase, move it to your questers, then during combat shift it back to your defender. While it can't cancel/respond to a shadow card, there is an action window between revealing the shadow card and determining damage, so there is an opportunity to change the defense value of a hero before applying any straightforward damage. Playing two-player, first player defends, then action window allows moving the warning to the defending hero of the second player; in a two player game, the next round with player order flipped, the move can be repeated if necessary.
Obviously these moves cost resources, so there is a definite opportunity cost involved and are probably prohibitively expensive if you're moving attachments a lot. But it's can be the difference you need to make or break a quest sometimes, since losing a hero or, to a lesser extent, failing a quest phase can prove critical.