I did not think about a trainer folder and sparring that makes sense.
Also on the whole weapon thing, almost anything can be a weapon. Some things take more creativity I guess, but I always considered a knife to be a tool first. I mean a lamp is a light source, if I hit someone with it is then a weapon. Although a lamp has a practical purpose, a light source. A knife is a tool designed to be helpful, if I attack someone it is a weapon. A gun for the most part is always a weapon, just not a "always active weapon". Unless you are extremely skilled you wont be opening letters with a gun. A gun generally has one purpose, to protect the wielder. Machetes and other large knives fall into that gun category where it loses some of its practicality.
I generally carry a knife, but as I ma not really a fighter, I would not say I carry it so much for defense as I do practical use. Also, know matter what situation your in, you need the ware with all to actually make a decision on what to do. So a knife is what you make of it.