[T. O'Heir]

The Gurkha Rifles in W.W. II had a philosophy about the use of their blades to remove sentrys with them making any noise. The Kukri is applied either vertically or horizontally to the sentry's head or neck respectively. Thus removing or bisecting the head entirely. Theory being that a sentry cannot cry out without his head.
It has both a psychological and physiological affect. The physiological affect is obvious. The psychological affect is primarily upon the guy who finds the missing sentry.
A blackjack or sap may have no affect of any kind. Even if you whack a BG up the side of his head, it may not affect him at all. And you need to get up close and in range of the BG grabbing you or otherwise causing you damage.
In any case, people tend to be frightened by blades more than blunt instruments because they're more familiar with what they can do.
"...criminals that do not care..." Mostly about whether or not said criminal is on drugs or not. Affects one's judgement.