I normally use a medium nib, which produces a crisp but fatter line: more colour, boldness, smoothness.
But for editing, it's not colour or richness I'm after. It's precision.
And it's curious how the tool unites with consciousness. I'm in a mood of resolve, brutality, coolness. No passage is so sweet or nostalgic that I can't delete it. No idea is so precious (to me) that it can't be cut out. The pen is less like a paintbrush, and more like a knife.
The extra-fine fits this mood perfectly. Still a smooth, lightly-wielded instrument. But it is severe. It suggests efficiency: of ink, space and language.
Of course I can edit without it, just as I can rewrite on computer, or with a rollerball.
But where possible, it's encouraging to have the right tools for the job. And, perhaps more importantly for anyone seeking mastery: to know the right tools when you see them.