Trust me. They aren't just for looks! My friend and I were playing at a CQB facilty in California and we both got knived. The guy had an MP5 K and was out of ammo, so he just slung it over his back and took out his plastic knive. We were reloading behind a corner, and then out of nowhere, BAM, we were both dead. It was the most insane way I have ever been killed in airsoft. From that day on, I have carried a knive, but never got a kill since. Who knows, maybe I will get lucky someday, but it is possible to get knive kills.
Good luck with knive kills,