Makes perfect sense. Somebody will be providing an ad blocker no matter what. If it is Google then they get to control how it works, why it works and when it works.
I instead thought "Other browsers will have ad blockers, so there are people who won't use Chrome if it doesn't have one." Implying that Google might consider that market segment and the browser strategy pretty darn important. Just a different take.
I think I agree with you. As an outsider Chrome looks like an application platform/sandbox for Google apps ('Google's AIR') that is just disguised as a browser. With their own platform they can move features more quickly than by fitting the apps into whatever box Apple/MS/Mozilla build. It could be the early MS playbook all over again. So...
Build whichever browser components remove barriers to entry for users (ad block, compatibility with many existing web apps, better performance/stability/ease of use, etc.) => build widespread adoption => commoditize desktop operating systems.