It's just a "being the number 1 guy thing". When you have two or three guys that are constantly throwing the belt around, there's no definitive number 1 and therefore title wins are cheapened (as they were in that Rock/Mankind feud).
Cena was extremely credible with the WWE Title in 2007, and if Orton had beaten Cena for the title at NM that would've been a huge deal. Much like if Rock were to take the title off Punk it would be a huge deal for that reason. Of course in Rock's case it'd be a huge deal cause it's Rock and it's his first WWE Title win in 10 years and his 10th WWE/World Title championship overall. But still it would look a lot better if Rock ended the year+ long reign of Punk as opposed to a one month reign of Punk if Punk lost the title and gained it back in the feud with Cena.
Going back to the number 1 thing, since it is going to be brought up, the reason the title is still in a rut is because Punk hasn't been "the number 1 guy". He's been playing second fiddle to Cena and therefore a win over Cena at this point means just as much if not more than a win over Punk. Rock already has a win over Cena. But if Punk can get by Cena and get all that momentum as the number 1 guy in the company facing Rock, it would make Rock's win all the bigger and more memorable.
But if Punk loses the title to Cena only to win it back, it cheapens the overall pay-off of Rock becoming champion... though that will still be a big deal regardless of who he wins it from and how long the guy has had the title, but why not make it the biggest deal possible?
And I don't know why people think a title change is bound to happen with Punk/Cena anyway. At NOC they could easily have Punk keep the belt by DQ or something, which would piss a lot of people off and get him a ton of heat in Cena's hometown. HIAC he could get a lucky fall ala HHH/Austin 3SOH. SVS... LMS? TLC is... TLC obviously.
Those write themselves in Punk finding ways to retain his championship. Or if they wanted to end the feud at SVS, have an ironman match which once again, could write itself. Punk comes out of the feud on top, Rock beats him for the title and does what pretty much every main guy on the roster and their mothers tried to do, but couldn't. Then Cena beats Rock at WM to bring the title back, gets his win back, and that writes that.
Personally though, even though I'd like to see Punk go a year as champion, I think it would be overall better if he lost the title to Cena at NOC/HIAC, Cena retains the title through the feud and loses to Rock. CM Punk then beats Rock to get the title back... which would I think be overall a lot bigger than holding onto the title an extra few months.
I just don't want to see the title switch hands more than once in the Punk/Cena feud, for reasons I already stated.
Punk as a champion for one year would be awesome. He would go down in history. Punk has come along way