Continuing with the value of blades a simple statistic here. Until the mid 1980's the weapon with the greatest number of kills in human history was the Gladius (The sword of the Roman Legions, which was modified and refined over the years.) This even trumped the standard firearms of the Napoleonic Era and the World Wars when total warfare with millions of casualties became the norm. What finally beat it out? The AK-47 which was in use throughout the war-torn regions of Africa and Central America by all sides.

Also a perfect line that is generally rehashed by anyone with the ability to use one - A knife is more dangerous than a gun. It never jams, never needs to reload, and little protects against it. The last part is the most telling, ballistic cloth/kevlar is resistant to pressure but is a woven fabric and as such is easily cut. To protect against a blade you need a heavier - generally metal or ceramic - plate to absorb the impact. This also results in reducing your own reflexes and reaction meaning you have to be even more careful around some one with a knife.