Taking It To The Street
MMA training is a lot of fun and can definitely be used to develop a formidable level of fighting ability and combat fitness. However, no matter how you slice it, once you take those skills off the mat or out of the octagon and put them on the street, things change drastically. A simple double-leg takedown or knee sweep that puts your opponent on the mat in competition is, for all intents and purposes, lethal force on the street. Similarly, a choke or lock that would qualify as a submission on the mat can leave you unconscious or crippled on the street and therefore defenseless against a follow-up attack.
Based on the real possibility of suffering death or grievous bodily injury when grappling on the street, the use of a weapon like a knife could certainly be justified in your defense.