Well, I admit I used to think like that. However, after some thought and age, I now realize that mindset doesn't make sense at all. If there is any place a self-defense weapon should be carried, ironically, it's in a place where a whole bunch of people are known to be without weapons. For every mass shooting and for every killer that preys on unequipped victims, this concept is readily apparent.
The nutballs out for a body count seem to concentrate more on heavily crowded venues. Whether the crowd has been disarmed on not doesn't seem to factor in. Just looking at the most recent incidents (Oak Creek Sikh temple, Aurora theater, Oakland Oikos university, and the Seal beach hair salon), most don't appear to be in traditional knife free zones.
http://timelines.latimes.com/deadliest-shooting-rampages/EDIT: I'm not advocating that carrying a knife is useless. A knife's day to day usefullness merits carrying one. I'm just arguing against the "unequipped victims" = likely target premise.