In its beginnings, MMA was a sport that matched style against style. However, in today's version of MMA, fighters must be well-versed in all the styles that the sport is comprised of in order to be successful.
If a fighter neglects training in any area, they are likely to have that hole in their game exploited by most, if not all, of their opponents. This is why none of the current UFC champions can be accurately classified as one-dimensional fighters.
In fact, all the UFC's titleholders would be described by many as the most well-rounded athletes competing in MMA today. These are the most complete fighters in the sport.