I'm with you on 1-3, but #4 seems wrong. What purpose is there to knowing when the comment was added if you don't know when the code was written[1]. This just looks like a trick you've developed to try to figure out when a comment is lying to you. But if you have so many comments that you doubt their veracity, you have too many comments. Read the code for truth.
[1] Without, as you mention, using your SCM system. But I'd argue strongly that if you can't do the equivalent of "cvs annotate" from memory, you're using the wrong system or you don't understand it well enough.
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.
[1] Without, as you mention, using your SCM system. But I'd argue strongly that if you can't do the equivalent of "cvs annotate" from memory, you're using the wrong system or you don't understand it well enough.