Nate Silver is a good example of the importance of writing well.
Silver is not a genius forecaster; his projections merely match that of other unbiased models. However, he writes much more clearly than other forecasters and statisticians.
I see parallels with my own life.
I’ve worked in IT for a long time now, and I am pretty good at it. However, I doubt I am in the top 10% of technical skill, and perhaps not even in the top 20%.
What I am, though, is a good writer. I write accurately and quickly and am capable of explaining very difficult technical concepts in terms that non-techies can understand (or at least have the illusion that they do).
I may not be in the top 20% of techs, but in the tech world I’d wager I’m in the top 1/10 of one percent of writing ability.
That has helped me immensely over the years, and is probably responsible for most of my advancement – that and interviewing well, which I suspect is a related skill.
The ability to craft a good sentence is a transferable skill. Unlike many skills, it helps you in every field. You might be able to succeed without it, but with it you do much better than you otherwise would even if you lack top-tier skills.