I disagree with the type of statement: 'using a knife substantially decreases its value' for a couple of reasons;
1- the value of a knife, to its user, is tied up in more than its monetary worth
2- a REAL valuable knife, ie; one that is made by an accomplished maker who stays in the business long enough to become known for his/her work, in the long term will increase in value, from a monetary standpoint, used or not.
Some collectors, maybe the majority of them, look at knives the way some collectors look at things like Beanie Babies, short term 'investments' which have little meaning outside of the status which holding them generates. Buying, selling and trading are what is really valued, the object itself, well I don't know whether it in itself is really valued beyond what it represents.
Some collectors look beyond their lifespans, or even years into the future, and see that a knife's value will never diminish. There is no compulsion to sell or trade it. Using it unleashes the knife's intrinsic values, either as a tool or as an artifact to display, depending on the knife itself. When a knife becomes an antique, well I don't know what that means exactly, but I do know that antiques are things that are generally highly valued, and have often been used during their time here on this planet. This use, quite often, seems to increase their value, (thanks, Antiques Roadshow:thumbup

.
Now, I don't want to make it seem like I believe it's so black and white, I'm sure that the majority of collectors do both of these kinds of collecting to various degrees and that there are other ways I haven't thought of. The human experience is a dynamic thing which allows for all sorts of shades of gray. There is no wrong or right when it comes to collecting, unless of course, someone is getting hurt by it, (Nazis, Silence of the Lambs, The Devil- they all practice 'malevolent collecting'

). The knife collectors I've met through this forum are, to the person, benevolent collectors

.