It's interesting that a similar question comes up with regard to watches too. I know some who own Rolex watches but wear "homage" watches that look the same, for fear of ruining the expensive watch.
I don't have many expensive knives, the most expensive was around $90, but like my watches I use them all.
There is a term that comes up with regard to wear and age called wabi-sabi, or just wabi. Supposedly it is a Japanese saying that means something like perfect imperfection. As an item gets used by its owner it gets better. So patina, scuffs, scratches and wear are all wabi, and make the knife more of a reflection of its owner.
Cool.