As with anything else, it's only useful if you know how to use it well enough. For me, a pen isn't all that useful, but a kubotan would be. A knife would be better, but I can't carry them often enough (as with the kubotan). A cane is better than a kubotan, less so than a knife, and I can literally take it anywhere I want. I'd be better served by a sword, but that's not legal anywhere I know of.