It takes time to customize a WordPress theme to make it “your own.” From simple color changes and replacing the header image- to massive rewrites of the CSS file, you can pretty much do what you want.
The theme switching plugin was built out of necessity for the theme review pages- so the host could upload all the themes into the theme folder- and then let the viewer switch from one to another to see what they liked.

Theme review sites:
http://www.alexking.org/software/wordpress/theme_browser.php
http://www.bloggingpro.com/wordpress-theme-gallery/
http://themes.wordpress.net/
http://www.emilyrobbins.com/how-to-blog
and this site which dissects the themes

Quickly, people started putting the code on their sites- allowing the user to chose how to view the site- here is an example: http://www.cafe789.com/kyles_eyes/
One note- make sure you install the switcher in every theme’s sidebar- or people will switch and get stuck.
I generally wrote off theme switchers- until the massive rewrite of one of my favorite news sites- www.macnn.com. The designer who rebuilt the interface is one of the most talented coders I’ve seen. His use of the themes switcher is what prompted me to rethink the value of this tool. He has provided a user “toolbox” to choose how to present the site- as a portal, news articles or headlines only- with your choice of type sizes.

The theme switcher seems to be built into the 2.0 interface so adding the plug-in will no longer be needed- but, the way you use it is up to you.