Fundamentally, Judo is enough to develop a person as a fighter and as an athlete. That said, it is important to find ways to expand your abilities. Fighting in something you are not comfortable will teach you about your habits and weaknesses. So even as a Sensei in Judo, I would always encourage my students to find other sports and martial arts to train in. Just do not get injured! Nor neglect your Judo training. If you are going to cross train, be smart!