I don't know that it's as much about Ziggler only being "good enough" for the U.S. Title (obviously, he's a main event/upper midcard guy now and a legitimate contender for the WHC or WWE Title at this point) as it is about finding someone to take the title from Ambrose without killing the Shield's brand. With all of the guys the Shield has run through, if he lost it to Zack Ryder or someone at that level, it would kill Ambrose's heat. If he loses it to a former WHC like Ziggler, it means something. Also, there may be some idea in WWE about rehabilitating the U.S. Title, making it mean something again, something along those lines, which means you have to give it to somebody who can elevate it. Ziggler carrying the U.S. Title and defending it a few times before being put into a feud to lose it (or voluntarily giving it up to pursue the WHC/WWE Championship, like when Goldberg gave up the U.S. Championship after winning the WCW World Heavyweight Title) could elevate the belt and give it more meaning.
There's also a scenario I've been kicking around in my head wherein Ziggler takes the belt off of Ambrose and "gets fired", thus being stripped of the belt and forcing the creation of a tournament (which, mysteriously doesn't have Ambrose in it) for the U.S. Title. Though this wouldn't use Ziggler to elevate the belt, tournaments usually help a title get a little more relevance.
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