I said this elsewhere, but it's relevant here too:
The idea that they might be giving Cena Ziggler's briefcase because doing so is somehow going to prevent Ziggler from becoming a credible, long-term main eventer really holds no water. Ziggler doesn't have to be a champion of any kind to be a main event talent. All he has to do is feud with the right people and either go over on them or else get enough 'rub' from them that he comes out looking like a winner even if he didn't win. This feud with Cena has gotten him as over as he's ever been, and even if he doesn't emerge from it victorious, he'll still be in the main event picture in some fashion.
Given that he's already lost to Cena once, I personally think he'll come out of TLC with the briefcase, but it won't be the end of the world - or the end of his relevance as a main event-caliber guy - if he doesn't.
I agree with what you're saying here, I also have to point out that the MitB briefcase does nothing for Cena nor does Cena bring any prestige to the briefcase. So, whats the point in taking it off Ziggler? He benefits plenty from the briefcase. Recently in an interview, a reporter mentioned that Ziggler's been a World Champion, Ziggler responded: "Technically." He went on to explain that he doesn't really count that one, though.
Sure, you and I agree he belongs in the main event, but I think he deserves more than (former one-time World Champion) being an afterthought when he is mentioned.