I don't like metacomments, especially if they are personalized like "I will add this later" or "Add the date if you know it", "if you are from London, join in", etc.
I think that in rare circumstances, they can be useful to the reader though. When I write about a topic and I know that I omitted a major aspect of it, because I ran out of time or I don't know enough, then I sometimes add a metacomment to the article, informing the reader of the omission. That way, they won't walk away thinking they got the full story. It's just a matter of honesty in that case, and it's especially important if people want to use the article as reference material.
Axel
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