You know what I call a nice example of "added value", the Burnout updates. No hassle, no serials, no fuss.
But the Burnout free expansions are a measure against the second-hand market! They give you an incentive to keep the game instead of selling it, because you know there is cool stuff coming up in a few months.
Our own experiments with downloadable (paid) expansions is that they do little in terms of sales themselves, but they revitalize the sales of the original game and help keep it full-price much longer than a game without expansions; I can see how free DLC would make sense.