All the good ones can. It might not be the job of the UX guy to do all the copywriting, but it is his job to make sure the person typing the walls of text is using the correct information, displaying it in a manner that benefits the goals of the product, and in the correct style so as to maximize conversions (or whatever metrics you're using today). A good UX guy/gal is a person who is adept at writing, designing, and hopefully at least a little programming. He/she should also have a great eye for detail and most important than anything else, be as rational as can be and use logic to define a projects direction in addition to one of the most important aspects of the UX field - user/a-b/whatever testing.