If you ask me, I like the way the knife worked in OFP:DR, had to select it out of your inventory to use it, had to left click to slash, and right click to stab, would work imo.
Plus, why are so many people saying having to use knifes in arma is rare? I have been in many situations where I could have stabbed someone in the back, in various Multilayer sessions. Ever strolled in the hills in an MP server, came across a sniper unaware that you are just 3 feet behind him? It happened to me once while scouting an enemy medical camp in a clan game.