I'm not a big fan of knives for defense either. Using a knife as a weapon requires a whole lot of training, and it requires you to get into knife range of the other guy. I carry knives as tools. Yesterday, for instance, I trimmed some posters to fit into a frame with my knife. I poked holes in the plastic on top of a frozen dinner. I cut a zip-tie on a power cord.
If I want to defend myself against a lethal force attack, I have a 9mm. If that doesn't work, I have a .38 Special as well. If that doesn't work, I should have brought a rifle. Knives have their place in the force spectrum, but it's very limited and they shouldn't be relied on as a primary defensive tool if you have the option of carrying a firearm. Hell, even a good defensive cane like the Cold Steel City Stick is better than a knife, IMHO.