Most writers can relate to the feeling that clients don't always see the value of their service. It's true that some potential clients consider "competent" writing – writing that follows the rules of grammar and spelling correctly – sufficient for their business needs. So how does a professional writer help the client understand the value of writing that takes that technical competence and uses it to target specific business objectives.
The key, I think, is to understand the discussion from the potential client's point of view. Strive to understand what's important to the client, and communicate in a way that is meaningful to them. Too often, writers deliver the "I am a strong writer" message, relying too much on work samples, awards or experience as the primary message. This is mostly meaningful to you, the writer, and your writer's ego.
Potential clients only want to spend money to meet their business needs (and can you blame them, really?), so you have to show how your way with words meets those needs. You must communicate how your service leads to results that they (not you) will be happy with. Focus on understanding their business and meeting their needs and your message will be much more persuasive.
Marta Kagan's "Bonafide Marketing Genius" Blog
Andre Sanders' "Running Without Condition" Blog
Jessica Sneeringer's "Mal-Diction" Blog