You can roughly make that case for any of the "core" skillsets of MMA - Wrestling, BJJ, Boxing and Muay Thai/Kickboxing.
4 of the current champs are BJJ blackbelts (GSP, JDS, Anderson, Aldo)
3 have elite Muay Thai and kickboxing backgrounds or skills (Anderson, GSP, Aldo). You could argue that Jones has excellent Muay Thai as well so that would be 4.
JDS was the Brazilian boxing champ I hear, and Cruz/Mighty mouse have primarily boxing centric wins even with their wrestling backgrounds.
I'd say you basically need all 4 of these core skillsets as much as possible, and the better you are at all of them combined, and the more well rounded you are, the more likely you are to be elite.
Most other martial arts are pretty watered down though and not really needed.. maybe a bit of Judo is nice but that's about it.