In my experience, it has been helpful in untangling contradictory changes based on changing customer specifications over time. Mostly, I think it's worth doing because the cost of doing so is really, really low (e.g. a hook to your add-block-comment function, a few characters), but there are edge cases where it's very helpful. Take it with a grain of salt, of course. (I edited #4 a bit.)

I agree with you about the annotate part, but (for example) I'm not clear if there's a cheap and/or fast way to find at what point a comment appeared in Perforce, when it may have been added six release-branch merges back. (I recently started using Mercurial for my personal projects. Night and day.)

There's a somewhat parallel debate about the pros and cons of signing comments.