"Henry Luce, a co-founder of TIME, disdained the notion of giveaway publications that relied solely on ad revenue. He called that formula "morally abhorrent" and also "economically self-defeating." That was because he believed that good journalism required that a publication's primary duty be to its readers, not to its advertisers. In an advertising-only revenue model, the incentive is perverse. It is also self-defeating, because eventually you will weaken your bond with your readers if you do not feel directly dependent on them for your revenue."
Time, "How To Save Your Newspaper", 2009
I keep thinking about this quote but forgetting where I first saw it, so I'm parking it here. It's a good thought; probably applies to a lot of web and mobile apps, too.