I think there is something amazing about someone who can write well. The way they can communicate exactly what they want to say, the choice of words....it is a true talent that often gets overlooked. Especially for those of us who are in the visual creative department.
First it is hard to find a dumb person who can write really well. They usually run hand in hand. That is not also to say that all smart people are good writers. But I would bet on the former as opposed to the later. So being able think and problem solve the issue of written communication makes you that much more valuable. At the end of the day, what are we as creatives? We are communicators and problem solvers. But instead of using sentences we use other visual means to solve the problem. So in my mind...it only makes sense that they would have a carry over from one to another.
Secondly, it is just an amazing skill to have. People pay big bucks if you can write well. Many of us are caught up in the kerning, tracking, white space....the photography, type setting and all the other things involved in putting together good designs. When it comes to communicating to others, especially clients....this is where we usually fall behind. It is tough to live just off your ability to design and have them speak for yourself. So it is important that we learn how to write so we can effectively convey what we have to say. At the very base of it, if you are missing out on this skill you simply leaving money on the table.