I always carry a sharp, lock-blade folder and I would use it if I absolutely had to for SD, but I don't think of it as an SD weapon for the following reasons:
- Good training for knife fighting is hard to find.
- Some jurisdictions frown on using a knife for SD in a worse light than using a gun.
- If you fight knife against knife, unless you are really good, and I'm not, you are going to get cut yourself, even if you are successful. What I am trained in is knife self defense involving controlling the knife hand/arm. I would go to that before I would go knife v. knife.
- I don't want to fight someone at knife distance. If I wind up in knife distance, my #1 goal is to get out of knife distance and not to stay at that range. I would create space to get to my gun, to run, or if I could not do either of those, to at least get to kicking distance. My kicks are good enough that I can dependably use them to do damage and minimize my own risks.