"The purpose of visual design is to facilitate communication.
When designing products that have a communication function, usable design is simply better design, because it makes a product better at its job. Usability is a central element to successful design. There is no either/or.
While it's certainly useful to test how successful a web site is, there's little point in discovering that your design doesn't work after you've built your product.
Testing can not give us new, creative solutions to design problems, or tell us how well a site achieves its non-functional goals. Excellence can only be achieved by building in usefulness in throughout the design process, and applying user-testing at key points to test success and to help polish the finer detail.
Why beauty/art isn't the new standard
What most designers mean when they debate 'design versus usability' is really 'graphic art versus design'. The argument generally revolves around the relative merits of aesthetics and function ('looks' over 'works').
Visual design is a discipline that applies graphical techniques to solve a communication problem, whereas Art uses many of the same skills for their own sake.
Art has its place and it can be part of great web sites, but creating a successful web site is all about design - finding the best solutions to solve communication problems.
Your web site can be both visually appealing and easy to use.
"There is a natural trade-off between functional and aesthetic richness.
You can't have something that is at the same time both an excellent high-functionality application and a great work of online art.
That point falls outside the sphere of design. The reason for this is that things that have the highest aesthetic beauty and impact cause you to stop and look at them, while things that are most functionally effective help you to do the job you want to achieve without being looked at. The two can't happen at the same time.
The most functional web sites are those that are information-rich, quick to load and totally obvious to use. While they can also be pleasing and attractive, their focus on function would be compromised if they were extremely visually impacting.
Likewise, the most beautiful designs - the ones that make you stop and stare - are rich in visually-stimulating elements. While they can certainly also be highly usable, they cannot also feature the weight of highly functional features that would also put them at the very top of the functional quality scale."