As AT&T attempts to swallow T-Mobile, one of the big carrots it's holding out for government regulators is the promise of universal, wireless broadband covering 97 percent of the nation. The company is right: this could be a big deal for millions of Americans whose economic opportunities have been limited by lack of good Internet access. But if that broadband is expensive and strictly limited, as mobile broadband is now, it won't make nearly as much difference.