two words; "clean code". How well coded your website is makes a huge difference to things like search engine performance and page speed. DW code view ends up leaving empty tags and stuff all over the place, i.e. mark up you just don't need.
For example, if you hit return in Design View, Dreamweaver creates this;
Basically an empty paragraph. OK, in the design it has created the space you want but what is it doing there? It is not semantically correct. Whereas a hand coder will make sure that the margins in the CSS handle the spacing so you just get paragraphs with stuff in them...As it should be.