IMO do what has already been posted in this thread. Do what puts a smile on your face.
For me that is using them. Be it a knife, a gun, or whatever. It puts a smile on my face to see a guy carry a cheap knife while he hoards a super steel super knife away in a drawer. But if that is what truly makes the dude happy so be it, it is his money.
I have a few duplicates of a few knives, mostly Sebenzas. Mainly because I had bid on and won more than one when I was looking. I almost never sell anything so I end up with duplicates/backups.
Out of curiosity I have wondered about people buying a backup knives. If you break a knife you were using in a rational manner do you really want another? Especially on your dime? I don't. I guess if one of my favorites was stolen/lost I would want another but I could probably find something else to use until I acquired another. Heck who knows by then there might be something else out I an drooling over. A newer model or better steel.
I am an accidental collector but I do not look upon myself as a collector. I am a user that gets a ton of enjoyment from my tools. Whether it be a knife, an axe, a firearm, or a wrench I enjoy quality tools being there for me when I need them.
I say unless it is something that is absolutely not replaceable use it. I use just about everything, I suppose if I were to happen upon an old rifle that was unfired I might save it that way. But if I could tell it had even one round down the tube I am going to shoot it. Now it is possible I might not carry it thru heavy brush on a hunt but barring that I would. Quality tools are meant to be used.