I've kept a rubber knife on me for the last 2 years of playing. In that time, I've had almost 10 kills with it. Mostly CQB. I've only had one kill in a open area because both our guns failed at the same time.
I'd definitely carry one, but don't expect to use it much. (Where you carry it is important. If you can't draw and stab in less then 1 second, don't bother.
Before the internet, the village idiot would stay in his own village.