If you're a good fighter and able to use your reach well then it's a very strong attribute. Shorter fighters can still reach the top though (no pun intended) since shorter arms have an easier time finding space on the inside.
Fighting is about technique, fortitude, intelligence, physical attributes, and being able to use all of the above efficiently. A short fighter won't win a war of jabs and oblique kicks against someone like Jones, no, but if they can find an angle they can blitz him. If reachier fighters where inherently better than the divisional champions would reflect this. Instead most of them - besides Jones who is on the tall end and Cain who might be on the short end - are about average for their weight class. There are advantages and disadvantages to both builds.
Reach is mainly advantageous in terms of stand up fighting. A number of champions rely largely on grappling for their success.
Jones and Anderson are two of the most dominant fighters in UFC history noted for their ability to out strike and both own a reach advantage over all and almost all of their opponents in their weight class, respectively. In fact, one of the few people Anderson fought at MW that had greater reach than him was Chris Wiedman.
You've got GSP but he was not successful due to his stand up to the same extent. He was dominant more so in wrestling and in mixing his skill sets together to win fights. His striking was a part of his game but not what made him dominate in and of itself. None the less, he owned a reach advantage over everyone he fought but Condit, Mayhem and Nick Diaz (whose reach is a known factor in his successful striking), and actually had equal reach with the latter two. Basically, he had the reach of a MW. Notably, he was able to out jab a number of his shorter-reached opponents to great effect.
Then you had Tim Sylvia who was a major problem to opponents due to his long limbs. Even when someone like Monson got him to the ground, he was able to utilize his great reach advantage to dish out a lot of damage from the bottom/guard with relative safety from Monson's GNP. That's the sort of thing a large reach disparity can give you.
Another guy known to be dominant due to his ability to Strike was Liddell. The only guy I could find that Chuck Liddell didn't have a reach advantage over that he ever fought was Allistair Overreem although there were a couple of fighters going way far back I couldn't find reaches for.
In the WEC, one of the dominant fighters and champions before he slowed down and lost his chin was Miguel Torres. For his weight class, his reach was huge and was a major factor in his success.
I would bet that most highly successful strikers have had a reach advantage and certainly not a disadvantage over a majority of their opponents. This is not a hard and fast rule by any means, but the trend seems rather clear to me, indicating how much of an advantage reach truly is in terms of stand-up striking.