Knowing your code is very important in controlling every aspect of your website, which is crucial in today's world of the web. WYSIWYG editors were good for learning what each thing does, but once you've completely grasped HTML and CSS you'd want to use a text editor to be able to understand what you're doing.
Another downside to Dreamweaver is that is adds unnecessary tags. For example, every paragraph would be translated as a <DIV> and your document will contain a series of class's with the name .style. We've not even touched on the left behind tags.
It's low level entry point combined with it's easy to use application made it a dream come true for wannabe web designer. As this is such an unregulated industry we naturally witnessed rock bottom prices overlooking the basics of web standards and browser cross-compatibility. Their technical expertise would be something similar to a person using Ms Word, but because of this low stepping stone this made it inevitable.
Many professionals with technical experitise just wanted to be in full control and the only way they could do this is if they dropped this paid program and opted for another editor. Many in the industry use a free HTML editor.
You're more likely in today's age to use CSS frameworks to do your hard work, such as 960, or the links I suggested above rather than using Dreamweaver.