The publicly available FBI statistics (that I can find) don't discriminate between different kinds of knives (i.e. kitchen vs hunting vs tactical - anyone care to suggest meaningful classifying criteria?) nor do they say what kind of violent crime (i.e. a mugger stabbing a victim in an alley with a kitchen knife vs a domestic disturbance that escalated to assault with a kitchen knife).
I don't see how any of this particularly relevant though with respect to the legitimacy or efficacy of state and local regulations.
Let's consider the domestic dispute in a home that escalates to assault which is often held up as the most common time a kitchen knife gets used in a crime. There are plenty of blunt objects in the home that could be used as weapons: rolling pins, hammers, baseball bats, and wine bottles. So why is it that knives get used as weapons several times more than blunt objects?
The answer is simple and obvious - Knives are designed for cutting and stabbing and are, by virtue of this design, more effective weapons (in nearly all situations) than blunt objects are. (And handguns, per the stats and by design are more effective than knives). As knife enthusiasts, we all know this already when we consider martial/tactical/self-defense uses of knives. Knives are weapons and some knives are more effective weapons than others, primarily due to blade length.
Let me put it this way.. if you were being attacked and didn't have access to a gun, would you pick up a hammer or a knife? A paring knife or a chef's knife?
Let's be clear... Regulations don't stop violent crimes but they do help limit the lethality of those crimes by reducing (not eliminating, but reducing) the number of the deadlier weapons in circulation.
It's a bit of a fantasy to play off knives as just tools and the fears that people have of knives as being irrational. But IMO, the statistics give average, non-knife people good reason to be wary of knives and particularly suspicious of people who use them outside of a "normal" context is. It's equally rational for states and towns with higher violent crime rates to regulate knives, particularly with respect blade length.