How often is this actually used?

Shoudn't it be called "deleted text", as the exact presentation depends on the style?

What are the use cases where this is actually required?

The proposed markup collides with the traditional use of "--" to mean a dash, and with the traditional signatures -- so it's not [CollisionFree].

-- [[RadomirDopieralski]], 2006-12-15

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Agreed.  It's useful in blog-like applications, but in a wiki it's just easier to delete the text.  I see no real use case for this, and it only serves to confuse people.

-- JanneJalkanen