I think it matters.
When fans and sports writers list what BYU brings as a candidate to the Big 12, I haven't seen the Yewtah rivalry listed as a resume` item, but does any of the other candidates bring a rivalry of this caliber?
The rivalry will be something that fans of Big 12 teams will start caring about to some degree. And fans of Pac12 teams will start caring about it because it will now matter for conference bragging rights.
The rivalry itself should increase in value.
Does any other candidate currently have a big rivalry with a member of a nonBig12 P5 conference?