• Ad networks can be, and are, used as a malware distribution vector. No one is vetting all the ads. Not the publishers, not the networks.
• They slow everything down. AdBlock and Ghostery make web browsing faster. Not to mention waiting 30s to watch a youtube video, or worse yet to realize that you didn't actually want to watch it.
• Ads are cognitive pollution. It's possible that mass marketing largely doesn't work, but I'm wary of assuming that "ads don't work on me." What if they do? As a matter of hygiene, it's better to avoid them. No, I don't avert my eyes away from billboards (ironic next gen Google Glass application) but I do skip TV commercials.
• Even if they don't work, they're at least distracting from stuff viewers actually care about.