Here's the thing...I think some people are thinking "knife in defensive situation" and some are thinking "knife fight!"
If a guy has a knife, I'd almost rather go barehanded, to better block and try to control his weapon, though a knife might deter an unarmed assailant. If you're outnumbered or otherwise justified in potentially lethal force, a knife might save your life. But you have to know what you're doing. If you start flailing around, you're not going to do much damage, and probably just get yourself hurt or killed. Especially with the folders most of us will be carrying. I'm not talking about bowies and military knives here.
A knife is the same as a gun in that regard...you pull it out, you better be prepared to use it, and to use it decisively. Face, neck, vitals. Arms and legs will hurt, but not stop a determined or drugged attacker.
Don't get me wrong, I carry a knife most every day. But I carry it as a tool first, defensive weapon second. I often forget I even have it when I get in a situation where I feel threatened.
Oh, and +1 on the above...great edge on the spydie.