If you believed single n-back was going to definitively beat the active control, then you wouldn't pay for a passive control. I buy that. But now that it hasn't, it's worth adding a passive control.
Some apparently randomly chosen training task (vocabulary and trivia memorization) exhibited just as much generalized training as single n-back. In your interpretation, neither had any generalized benefit, then - the improvement is just due to normal ~9yr old child development over the timespan.
I do recall hearing some credible evidence that dual n-back (whatever configuration was in some older Jaeggi study) gave a boost to "fluid intelligence". (thus the interest in the topic). But now I'm given to mistrust Jaeggi more than I would the average influential researcher.
Following up on the 2010 study, Jaeggi and University of Michigan people have run a Single N-back study on 60 or so children.
The abstract is confident and the mainstream coverage unquestioning of the basic claim. But reading it, the data did not seem very solid at all - I will forbear from describing my reservations exactly; I have been accused of being biased against n-backing, however, and I'd appreciate outside opinions, especially from people with expertise in the area.
(Background: Jaeggi 2011 in my DNB FAQ. Don't read it unless you can't render the above requested opinion, since it includes my criticisms.)