I've seen an awful lot of injuries caused by both gunshot and knife wound - and knives scare me a whole lot more than guns! A gun will typically make a nice round hole, and unless it hits something really important, you've got a pretty good chance of surviving without long-term damage. A knife can cut and slash its way through organs, blood vessels, ligaments and tendons, etc. There's a much higher possibility of permanent damage - or death from blood loss - if you're carved up by someone who knows what he's doing.
Working in a prison as I do, over the past couple of years I've seen plenty of knife wounds (and helped to stop more than one knife attack). Untrained users typically stab wildly, and the results, while messy, are usually survivable. However, when the operator knows what he's doing, even with a homemade shank without much cutting/stabbing power, the victim is more often than not in a whole heap of trouble. It's also remarkably quick - no more than a few stabs, over a few seconds, can kill or disable very effectively.