I think the problem is that most people don't understand that knitting and yarn collecting are actually two seperate hobbies. They are related, and most people that knit also collect yarn.
I've never heard someone that collects baskets say "oh, I'll never use all my Longaberger baskets". Of course not, because the idea behind Longaberger baskets is to collect them. Why can't we do the same thing with yarn?
I get the same pleasure buying yarn for my stash as someone that collects any other item. I know comic book collectors, Longaberger basket collectors, Hummel collectors and even spouse collectors. I like buying yarn simply for the sake of having it. To display it, to touch it, and yes, maybe even to knit it. Okay especially to knit it. I love planning what I can do with a yarn. Choosing colors/patterns/needles, etc is one of the reasons I knit. I like the idea of the practicallity of my hobby- turning something I've bought into something useful.
So embrace your yarn
Oh, and yarn doesn't need to be dusted.