Like Piskie, I often do copy editing and copy writing because once a file's sitting there on my desktop, I am going to fix everything: copy, code and graphics.
I do too. But because I'm not a copywriter (even though I probably know more than the avg. joe), the times when I just edit or try to write my own copy, those are the times when I can run into trouble with clients (and rightly so). It's just much better/smarter to build the price of professional copywriting into any job. Of course, some larger clients have their own copywriters on staff, so that helps as well.
Let's face it, SEOs do not make the best copywriters, and copywriters do not make the best SEOs. There are those who think they can do both, and do try, but it's not really in the best interests of clients to work that way, imo.