1. Rematch Value
One of the big reasons for having Cena go over Rollins at SummerSlam is that it instantly sets up Cena Vs Rollins 2 at Night Of Champions. There wouldn't be the same value if Rollins retained at SummerSlam, there needs to be a reason for a second match. Cena being the champion is the role reversal which gives the feud renewed life for another month worth of story telling. Cena would of course retain at Night Of Champions, going on to work with other stars for the remainder of the autumn season. Rollins meanwhile would start his feud with Kane, which you'd expect to conclude by the time of Survivor Series. After that, one rumour is that Rollins will turn babyface. There's even talk of him winning the Royal Rumble and going on to fight for the WWE Title at WrestleMania 32. So it isn't all doom and gloom for the 29 year old, his time as champ may be up, but his future is continuing to look bright.