I don't think he should lose the MITB briefcase. It's an unnecessary setback that Ziggler doesn't really need. Ziggler "failing" to cash in his briefcase only serves as a great tease. Whenever it's looked like he's about to cash in, the live crowd goes nuts. The "any place at anytime" aspect of the MITB concept is a HUGE part of what's made MITB so successful.
With WWE teasing Ziggler cashing in, it leaves the door open for them to potentially go in any number of directions with him. As long as they don't overdo it by having Ziggler tease cashing in so much that it loses it's novelty, they'll be fine. I think part of the problem some people are having is that they're still unused to the MITB briefcase holder keeping the case for extended periods of time. With the exception of a few times, most winners cash in on their first attempt.
Ziggler's popularity continues to grow. He gets a good mixed reaction these days, we saw examples of it during his feud with Jericho with dueling "let's go Ziggler/Y2J" chants during their matches. I don't think they'd have any problems turning Ziggler face and, personally, I think there's potential money in it. He's been with Vickie for a long time now and, frankly, he's long since gone as far as he can with her. Before this year is up, I expect Sheamus to drop the WHC to Alberto Del Rio. The fact that Del Rio scored not only a clean win over Randy Orton last night on SD!, but also scored a clean submission victory, suggests that WWE might still be punishing Orton for his Wellness Policy violation. Not that long ago, it looked as though WWE was teasing a possible feud between Del Rio & Ziggler. They had a couple of physical matches in which neither one of them came out on top, Del Rio kept Ziggler from cashing in his MITB contract very early on, they had a few heated words inside the ring, etc. I don't think it'd be difficult at all for WWE to turn Ziggler face, build him up over the course of the build towards WrestleMania and have him cash in his MITB briefcase at WrestleMania 29.
With the way the WWE Championship scene MIGHT be playing out, I think this is at least a possible way to go about things. Based on reports, Vince is pushing for Cena vs. Rock II at WM, which will almost certainly be for the WWE Championship. If that's how it's going to go, then I expect Punk to retain the title during his feud with Cena before dropping the title to The Rock at the Royal Rumble. Cena could be a participant in the Rumble, wind up winning the thing, and go onto WM in an attempt to avenge his loss at WM 28 and regain the WWE Championship. Ziggler, whether a face or a heel, could keep his briefcase and cash in at WM for his shot at being World Heavyweight Champion.
I know a lot of people don't like the idea of Cena winning the Rumble & going on to face Rock at WM for the title. I'm not personally thrilled about it but, let's face it, it's probably going to happen just that way if Rock's schedule allows him the time to compete at WM. So, this keeps the briefcase with Ziggler, the WWE has LOTS of time to determine what direction they want to go with and Ziggler could finally wind up having a great WM moment of his own.