What should the advantage be to nick = _compared to just not defining a nick and let the client use the domain as the handle?

What is not intuitive is that you put something in the nick field that is not to be taken literary. The special meaning of _ is only clean if you read the documentation, compared to having something in nick that makes sense in the current context of the twtxt.txt.

You are right: no advantage. Also your method can make backward compatible to feeds which doesn’t implement metadata extension