to be a proficient mma fighter? I dont think you need to know gi bjj, I dont even think you need to be an excellent subgrappler, you could be a striker with good tdd and subdefense, that has proven to be more than enough to excel at MMA, of course there's the other way, where you can be an exelent sub grappler an not be an awesome striker, and that has proven to be more than enough to excel in MMA too... there are different roads, many people have their opinions on this subject, I personally dont agree with the say of training in the Gi makes you a better grappler, then again, the biggest name in the grappling world said the contrary, then again you have awesome subgrapplers who have never trained in a gi coming from catch, LL and other no gi based systems...