Striking and Grappling are the two main principles of MMA. Anything other than that is your personal style, and thats the point of MMA.
Im actually getting tired of seeing people getting into MMA, who just entered a gym and said alright Ill take one wrestling course here, one BRAZILIAN JJ course here, and one Muay Thai in my afternoon class and hope to get into UFC. No, thats stupid, thats like the industrialized, corperate way of becoming a Mixed Martial Artist.
However, its nice to see someone whos been doing Judo, or Karate or Sambo, get in the gym, and enhance their striking/grappling well enough that in addition to their original style, can compete against other guys with different/similair styles. And that is essentially the root of MMA. Guys with already established backgrounds, enhancing their other not-so-great attributes and competing in Mixed Martial arts competitions.
In the end you really only need to know how to Strike, Wrestle/Manuver your opponant into better position, and be able to submit dudes when the fight hits the ground, thats a generic MMA fighter for you. But a fighter that is proefficient in another style, in addition to the general 3, well thats a true fighter for you.