The debate never ends. Some argue that all of these allow gamers to get 'free goods' without buying and thus negatively impact sales (the 'Why give away the cow?' crew). The proponents argue that these things ease people into purchases, attract more gamers to the fold, and let gamers spend more dollars because they feel their dollar go further (the "Spread the love" crew).
The argument comes up in discussion of rental or subscription to retail games as well as in discussion of free web-based casual games and other casual games distribution mechanisms.
I've blogged about this debate before, and how other industries might give us pointers.
On that note, two related pieces of research popped onto the radar today:
For the Nays: This piece of research on radio's effect on music sales, concluding that radio play does NOT boost sales of recorded music, and in fact may do the opposite. (As a side note, I am personally VERY skeptical of this peice of research. It seems to jump very quickly to support an argument for music licensors charging license fees to radio stations for airplay. I do wonder whether it has some bias behind it.)
And in the other corner: This post from Freakonomics on Libraries. While it doesn't argue that libraries are good for book sales, it does point out that books still sell despite them. It also asks out the provocative question "If libraries didn't exist, could you start one today?", concluding that the publishing industry would likely attempt to quash such an effort or at least shackle it with a license fee structure. I think most would agree that libraries are good for society and good for culture, and that they haven't yet killed the publishing industry. (Oh, and be sure to read the comment thread. There are some links to a number of cool things including some examples of people that ARE in fact starting libraries today, on the web)
Anyhow, there you have it. Arguments to back up whichever side of the debate you are on. Please ignore the other one.