The visual design process is massively important to the success of a web application. If the look and feel of a web application is not right then users will not be inclined to maximise the power behind it.
The key difference between web designers and graphic designers is that a good web designer will implement a design that conforms to every standard available, ensuring that the site is maximised for cross browser compatibility and SEO attractiveness from the outset not as an afterthought.
Dedicated graphic designers are a valuable tool in the initial stages of good web design, at Shore Tech Systems we have a pool of graphic designers who can be involved in the initial design and layout of any web application.