I occasionally get into arguments about the questionable ethics of blocking advertisements on web sites. Usually it's compared to piracy and the argument falls apart. The end-result is getting something for free and the publisher getting nothing, but that's where it ends.
One of the key difference is that (short of a few nasty DRM techniques some publishers insist on using), it's usually the pirates that have to worry about malware. Failing to block ads works the other way.
By failing to block advertisements, you're allowing a trusted third-party (who is usually using fourth-party) to serve you ... software. Software that you hope doesn't result in your computer becoming a zombie. That's trust that should only be given to the most savvy of third-party advertising networks, and the most savvy of publishers.
For a long time, if you didn't visit pr0n or warez sites, avoided P2P piracy or Usenet alt.binaries, and kept your AV up to date, you were unlikely to encounter trouble.
Such hasn't been the case for the last few years. Today's story of trusted third parties delivering malware comes from Google.