That was my main problem with the match, although, I guess for different reasons. For me, it just didn't make sense that Reigns would involve himself in the Cena/Rollins match
1. He has no allegiance to Cena
2. Why would he care to preserve Cena's title match?
3. Yes, he has a personal grudge against Rollins, but that doesn't explain why he would interject to hurt Rollins because (a) Rollins was not going to get the number one contendership automatically if Cena lost (b) even if Rollins WERE to get that contendership and Reigns cost him it, it wouldn't matter since Rollins still has the MITB contract so he wouldn't have really lost anything over it
So basically Reigns' only accomplish was insuring a title match for a man who has probably had more title matches than anyone in wrestling history
Orton would have made more sense, I agree, simply because he was burned by Rollins more recently and he might have been closer in line to the title match if Cena lost. Though, technically, I guess Orton really didn't have those motivations either.
Either way, it was just a nonsensical ending to that match.