The security argument is fairly good in the sense that the extension security model is broken. It's not even about ad blockers: far too many extensions request overly broad permissions and then either do sneaky things (e.g., "monetizing" users by stealing browsing histories) or put users at risk. It doesn't help that if you pop a developer's account, you can essentially deploy a backdoored extension to all users *instantly*. 

But, there are many ways to improve this, and Google has chosen an approach that is inherently controversial given that they're an ad company and that their other divisions are openly waging a war on ad blocking right now.

/mz