The ability of a Jujitsu black belt to win in the UFC cannot be disputed.
One of the most important aspects of MMA is knowing what to do when the fight meets the ground.
The Gracie family played a large role in the beginning of the UFC. When the UFC began it was a no-holds-barred competition of strength and ability.
Rules? There were none. Weight classes? Unimportant.
And who won many of those matchups? You guessed it—Jujitsu experienced fighters.
According to Wikipedia, "With no weight classes, fighters often faced significantly larger or taller opponents. For example, Keith "The Giant Killer" Hackney faced Emmanuel Yarborough at UFC 3 with a 9 in (22 cm) height and 400 lb (180 kg) weight disadvantage. Many martial artists believed that technique could overcome these size disadvantages, and that a skilled fighter could use an opponent's size and strength against him; with the 170 lb (77 kg/12 st) Royce Gracie winning three of the first four UFC events, the UFC quickly proved that size does not always determine outcome."
Many fighter's have black belts in Jujitsu or are experienced wrestlers because those are the fighters that are able to win and keep contracts. Yes there are a few Jujitsu-experienced MMA fighters who do not do well in the UFC, but the fact that they had to win professional fights to be in the UFC cannot be ignored.
Pound for pound, stand-up-only fighters do not always have the upper hand—for example, Mirko Filipovic and Chuck Liddell. And yes, Quinton Jackson, arguably one of the best fighter in the UFC, is one of the pound-for-pound, stand-up-only fighters who does very well, however, he has a rather extensive wrestling background. If the fight hits the floor he knows what to do.
There is no evidence to support having an extensive background in some form of karate or certain other martial arts will produce more well rounded fighters than those of the Gracie and/or Machado Jujitsu camps. Jujitsu, wrestling, and boxing remain three of the most important aspects of fighting, and any fighter who has an extensive background in one and trains in other areas has more than a fighting chance of reaching the top of the UFC and MMA world.