On whether or not knives may be used effectively to fight with:
Of course they can. Which is not to say that all people, all knives, or all supposed "knife fighting" techniques lend themselves well to the prospect.
For example, someone who knows his way around a boxing gym would be a devastating opponent in close with a push knife in his fist. It would be fighting with a knife, all right, and damned right it would be effective, even without much resembling what many consider to be "knife fighting."
Or another, this time with a mixed martial artist looking to ground and pound someone having introduced a conveniently located small fixed blade. Again, not much like "knife fighting" in the minds of many, but effective? Yep.
But, perhaps even more true than Cooper's quote would be it adapted for knives:
"Owning a knife doesn’t make you armed any more than owning a guitar makes you a musician.”
Meriam Webster's: Main Entry: ci·vil·ian Pronunciation: \sə-ˈvil-yən also -ˈvi-yən\, Function: noun, Date: 14th century, 1: a specialist in Roman or modern civil law, 2 a: one not on active duty in the armed services or not on a police or firefighting force b: outsider 1, — civilian adjective