If I have to use a knife, it's going to be a quick, decisive, deep and thorough incision to protect my otherwise worthless hide.
It's hard to make a quick, decisive, deep and thorough incision without getting cut yourself. In fact, if the other guy is any good at all, it's impossible.
It's possible to cut someone without getting cut, but generally only on the hands, arms and head (people tend to lean forward at the waist).
I usually have a knife or two on me. Here's the one I ALWAYS carry, but I usually have something larger tucked away as well.