The typical way a brand's performance on Twitter is judged tends to be by their number of followers, engagement via mentions and RT's, or Klout score. When a brand commit a "Twitter fail" it's often down to a one off instance, such as someone "dropping the 'F' bomb". Half the time these instances can be passed blame on miscreants obtaining log-in details or careless uses of TweetDeck between personal and company accounts. But what goes un-aired are brands who are simply using Twitter wrong. You could call it an opinion of what's considered using Twitter correctly, but for brands the be-all and end-all is to increase engagement = increase community = increase sales.
Every Tom, Dick and Harry that have started their own Social Media Agency has written an e-book on How Brands Should Use Twitter, students are writing their dissertations on it and companies are hiring their staff accordingly. However, some seem to be missing the bigger picture.
Brands that retweet every single mention about themselves is the equivalent to people loudly repeating every compliment they ever received. It's generally annoying and can be perceived as arrogant.
They may not know it, but they are deterring potential audience members from giving their time and attention to the brand.
It would be ten times more effective to continue to generate conversation with that user, or even better, reward them. If you engage with that user enough that they'll retweet your message to their followers you know you're on the right track.