The only "grudge" I can think of nowadays is Matt Hughes & Matt Serra (they're fighting at UFC 98). Besides them, everything else is usually just to build hype for a fight.
MMA is not a work (predetermined) like pro wrestling is.
As for Leites/Silva....Anderson Silva is one of the most dangerous strikers in the UFC so if Leites tried to stand and trade punches/knees/kicks with him, he would have been knocked out.
Thanks for explaining!!!