[adsr]

Another point is the fact that mobile Safari wont allow the use of plugins. A wild guess is that ad blockers are the most common plugin that people use, so it doesn't seem unreasonable to add this in the browser itself as an opt-in feature.

I don't really see the difference from using an ad blocker in the form of a plugin, and blocking ads by turning on the feature in the browser itself. Ad blocking is also a feaure built into the latest Firefox release.

[planb]

But Safari in iOS9 doesn't have an ad blocker, it provides an interface for extensions to efficiently implement content blocking.

[bitserf]

One potential weakness is that a declarative rule list controlled by the browser (iOS 9) would be slower to adapt to ever changing approaches use to bypass ad blockers.

Whereas the extension model of AdBlock I presume lets it be more clever.

But on the flip side, the Safari approach would be much more performant than having to execute some JS to enforce blocking.