Hm, I'd say the reason why John Cena is doing this programm with Del Rio now is not even so much that Del Rio needs Cena to put him over... but the fact that if Cena wasn't working with Del Rio now, there would be nothing to do for Cena.
After all, the OTHER high-profile feud on RAW right now is going to be the Punk/HHH/Nash triangle and however that will play out with the potential involvement of maybe Stephanie and/or Vince McMahon or a potential other third party - and I do believe that this will in fact be the MAIN storyline for the RAW weeks to come.
On the other hand, who is left for Cena to work with? It could be argued that he could work with Ziggler, who is still a heel, but I suppose his program with Alex Riley is far from over, and besides it would de-mote Cena to mid-card status (feuding around the US-Title), which WWE will certainly NOT do. So the ONLY other main event calibur guy to feud with Cena right now is Alberto Del Rio, alone for the fact that he has the WWE title, which makes him a main event player by default.
But besides ADR, there really isn't a big enough heel on RAW right now to justify feuding with Cena, as another Cena/Miz feud is out of the question after the LONG program they had leading into WM. R-Truth/Cena has also been touched upon already, and then there's not even much mid-card left anymore for Cena, let alone main event calibur. Sure, they could put Morrison against Alberto Del Rio right now, though that would just be another case of Morrison getting buried, because there's no way WWE are going to strip ADR of the title this soon, especially not by having him lose to a not yet established main event guy, but more of an upper midcarder like JoMo.
So the only logical thing is to pit ADR against Cena now, have Cena put ADR over as a REAL main event player, and then probably JoMo will get his shot... he will still most likely not go over ADR, but once ADR is a more established main event player (which can by achieved by a program with Cena), it will be less degrading for JoMo if he loses to ADR. Eventually, I suppose either Cena or CM Punk will get another chance at ADR's title around Survivor Series, with Punk either getting the WWE title involved in his angle with Triple H & Nash, or Cena regaining the belt and maybe feuding with Ziggler (if they want to launch him into the main event picture in a few months after his current US title reign) to get warmed up for the Road to Wrestlemania.