Pfft, Cena already gets enough cheap pops by mentioning other wrestlers, like Daniel Bryan, in his promos, and sticking him in the Shield would be a pathetic way to try and force the fans to cheer for him. Besides, they are supposedly trying to slowly build Reigns up as a monster face, and you put Cena in that faction, he automatically is the biggest star in it, and Reigns would be a tougher sell as the top face they eventually want him to be.
Also, who said Cena can't work or doesn't put people over?
Ehh...I thought about the Reigns thing too. Here's the thing though.....traditionally, you can't build a monster babyface without them being a very significant heel first. Rock, Austin, HBK, Triple H, Undertaker, Batista, Eddie Guerrero, Punk...even Daniel Bryan were all pretty major solo heels before their huge babyface runs. The only time I've seen otherwise would be with guys like Hogan, Bret Hart or Goldberg. Also, Rollins, Ambrose and Reigns are still VERY young in the WWE. Rollins aligning with Triple H and Orton is a very smart booking move because this will SIGNIFICANTLY push Rollins into the stratosphere. The Shield is still too young in the business to elevate someone even younger (I LOVE Sami Zayn btw). I feel them pushing Reigns into a major babyface role might be a little presumptuous. Who knows though?
Also traditionally....anytime ANYONE has aligned themselves with one of the 5 biggest names in the history of the business (Hogan, Flair, Rock, Austin, Cena), whether it's in a faction or a feud, they get elevated. Hence what's happening with Bray Wyatt currently. I just think it would be a significant shake up for a character that should have been shook up 5 years ago. I'm still pulling for the 3rd man to be Ziggler though
They are completely wasting him.