I've repped my rubber guard while rolling from day one at my gym, and my instructor has never stopped me or said to focus on other things because he has an open mind, and because he sees me working over some of his most promising students in my guard, I say with all humility. You need to put in mad reps to be truly good at anything in life, not just jiu jitsu, and the wider your repertoire the more possible options you have to end the fight and stay out of danger. Because of innate flexibility, I've been able to dive into rubber guard from the beginning, but that won't work for everyone. The fundamentals of Judo, Wrestling, 10th planet JJ, Kung Fu, and many other arts work for anyone able enough to put their reps in. In the end its what you do and often how many times you've done it before that will determine your performance in a given situation.