Hope nobody missed me. Been out for a week due to surgery, good times. Still recovering, but I found I'm finally lucid enough to try to catch up.
Honestly, I think people are also overlooking something simple: If someone has a tendency to show up to fights using the magical equivalent of a tacnuke, they're going to get NOTICED. Any enchanted item must be carried in order to be used, and will be trivial to notice/find by even half-assed practitioners.
People will come after you for your gizmo, either to have it themselves, to keep you from having it, or to keep someone ELSE from taking it from you. And since you're a one-trick pony, it won't be hard for someone of sufficient determination to do so.
The story will quickly become about the item, keeping your control over it, and surviving the attempts to keep it. This can be both used as a way to justify the item (perhaps everyone enjoys the edge of the seat 'omg, the fight came to us again, how will we hold onto the mystic amulet of pwnz0r this time' action), or as a way to get the powergamer to get sick of "I've LOST my wadget AGAIN?!" and think up a more clever approach.
Not to mention a weapon of that strength is probably going to get aimed at a mortal at some point, and we already have a huge stack of tools for dealing with THAT through story.. ;)
I might not want something like this in my game, either, but I think trying to scramble to 'patch' the 'hole' is kind of understating the capacity of the game to self-correct without more fiddly rules. :)
As an exercise in curiosity for everyone, if you were going to design a 9 refresh offensive item of power, what would it look like?