Free content, sure. But also ads for penile enhancements and dialog boxes that go "Congratulations! You've won $100,000 in Amazon gift cards! Click here quick [obvious phishing link] to claim your prize!". Scams, pure and simple. Not claiming that tracking prevention is going to solve these problems, but any move that at least attempts to reign in the worst excesses of a scummy industry is a step in the right direction IMHO.
Anil Dash had a good thread about this recently, and he puts it much better than I: