I think those that are asserting "it's not a knife if it's not being used" have backed themselves into a strange position in the debate. Does it have the attributes, construction and materials typical of a knife? Could it be used as a knife? Then it's a knife. Call it a "potential knife" if that makes you feel better otherwise you're just making yourselves look silly.
+1. Well said.
There's plenty of things that can be an industrial design item, meant for mass production, that transcended that fact and became art. The items become both art and the item that originally names what it is.
Take Bob Loveless's knives for example or more widely accepted for its form and function as well as a piece of art the Eames Chair by Herman Miller. Can be viewed at the MoMa
Great point... I was thinking of Loveless knives as well. Given their worth, some of his last knives may never have cut anything more than paper, and probably never will; but suggesting that a Loveless isn't a knife would probably trigger apoplectic fits, grand mal seizures, and spontaneous combustion amongst more than a few BF members.
And I wasn't going to get into it, but since you made the point so nicely: Definitely. Knives can be art as well... both actual f***ing 'Art Knives', which qualify as folk art or even outsider art (Rick Hinderer -- the hard-use hero and one-time first responder -- got his start doing art knives); and high art as well. One of the most important artists in history used knives for his most recent conceptual art masterpiece.
I can only be referring to James Franco, of course -- and his mind-shattering brilliance

-- as well as his work dedicated to his (now-deceased) pal Brad Renfro. The two met while filming the unspeakably awful 'Deuces Wild', about gangs in 1950's New York... therefore, switchblades.
As someone obsessed with art both historical and contemporary, high and low, I feel wrong mentioning one of the many douchebags who's done his part to make contemporary art a joke to the educated public, along with Bjork, Jeff Koons, Damien Hirst, and 'Artnews' (with its new influx of 'talent' no doubt harvested from the crop of TMZ cast-offs).
Maybe one day he'll make a real contribution, and end up in one of Hirst's huge installations, filled with formaldehyde and hugging a dead shark.