There are two cases where the Design Wizard would be beneficial to an established merchant:

• If you intend to open an additional store in a niche market, you can use the Design Wizard to quickly select a design for your new store. This will give you a good looking site which you can open for business with and test your second store in the market without having to spend excessive time customizing the store just to open for business. Likewise, if you typically have custom templates for your existing store(s), the Design Wizard allows you to get an updated look for your store without the expense of hiring a designer.
• If your existing store uses the standard base templates with only edits made to the Variables, then the Design Wizard is a great way to update the look of your store. The new base templates use full CSS, offer many more variables for controlling most of the aspects of the store design, and feature many SEO best practices.

For most merchants with custom templates, using the Design Wizard is not advisable. Completing the wizard will install a new set of base templates in your store and completely delete any custom templates.

While the new templates are a great improvement over the base templates in terms of more standardized clean coding, typically fully custom templates include features in excess of the base templates such as pagination, multiple navigation menus (browse by price, brand, etc), multiple product images and others. Those features are not present in the base templates and would need to be added onto the base templates by copying and modifying them.

The new base templates are run on an entirely new code base so it is not possible to use both the new templates while being able to edit your custom templates.

The new templates will not be installed unless you complete the Design Wizard. This means you can enter the wizard and see what is available. If you click the Finish button on the last screen of the wizard the new templates will be installed so you will not want to do that unless you do wish to move to the new templates.