Of course he'd be better off. But sometimes you've just gotta take a shot. That being said, Cena winning and then dropping it to our Viking Space Lord is the biggest money match for Summerfest, especially with Reigns over HHH as the semi. Cena can carry the belt, it doesn't hurt him to lose right away, and he owes Brock a job anyway.
OK, as much as I'm a Dean Ambrose fan out of the trio, I can see Roman Reigns being THE guy for the next few years, as he's really improved in the ring and has connected with the fan base. However…
WWE history shows that people that win the championship for the first time in A. fluky ways like MITB cash-ins (Miz, Ziggler, Swagger) or B. multi-person matches (Mysterio, Big Show) really have flat reigns and doesn't give that initial title reign any muster. When the person is built up on a mission to win the title (Eddie, Bryan, Austin, etc.), the fans EAT IT UP.
So don't you think that Reigns would be better off winning his first title in a traditional 1 on 1 match at a major PPV? We can guess that whoever wins MITB is getting fed to Bork Laser at Summer Slam, so why make his first title reign a short one, when defeating Borklestein would give him 10 times more cred?