I'd make it a Wildcard thing only - players or bosses that should be rivals to the players. I'd reset it on the next turn in which they were able to take action. So: if they recover with Raise and are able to take an action, that resets the +2 cumulative bonus. if they recover with no Raise, but are immediately Shaken again, they retain the cumulative bonus.

Fair enough re: wanting to buff weapon damage. :)