Web ads considered as a security exposure
One of the things that reading Twitter has exposed me to is a number of people who deploy browser adblockers as part of their security precautions. This isn't because they're the kind of person who's strongly opposed to ads, and it's not even because they don't want their users (and themselves) to be tagged and tracked around the web (although that is a potential concern in places). It's because they see web ads themselves as a security risk, or more specifically a point of infection.
The problem with web ads is web ad networks. It's a fact that every so often web ad networks have been compromised by attackers and used to serve up 'ads' that are actually exploits. This doesn't just affect secondary or sketchy websites; major mainstream websites use ad networks, which means that visiting sites normally considered quite trustworthy and secure (like major media organizations) can expose you to this.
(As an extra risk, almost all ad networks use HTTP instead of HTTPS so you're vulnerable to man in the middle attacks on exposed networks like your usual random coffee shop wifi.)
Based on my understanding of modern sophisticated ad networks and the process of targeting ads, they also offer great opportunities for highly targeted attacks. At least some networks offer realtime bidding on individual ad impressions and as part of this they pass significant amounts of information about the person behind the request to the bidders. Want to target your malware against people in a narrow geographical area with certain demographics? You can do that, either by winning bids or by hijacking the same information processes from within a compromised ad network. You might even be able to do very specific 'watering hole' style attacks against people who operate from a restricted IP address range, such as a company's outgoing firewall.
(The great thing about winning bids is that you may not even be playing with your own money. After all, it's probably not too difficult to compromise one of the companies that's bidding to put its ads in front of people.)
If you're thinking about the risks here, web ad blockers make a lot of sense. They don't even have to be deeply comprehensive; just blocking the big popular web ad networks that are used by major sites probably takes out a lot of the exposure for most people.