What you are talking is not a responsive design then, you are talking about a fluid design using percentages for widths and Ems for font sizes.
A responsive design is one that uses different settings based on the response to predefined criteria.
The most common example is viewport. The drawback to using viewport as a response is that it forces a desktop browser to use mobile device settings if the browser is resized accordingly.
Using WURFL means that you can have a separate layout for the desktop and one for the mobile device and the mobile design will never be imposed on the desktop.
It allows you to offer a completely different form experience based on the device. The mobile browser does not support file uploads for example, you can have a contact form that on the mobile device does not include the file upload, but on the desktop it does. And the desktop form will not be effected by changing it's size.
Those are just a couple of examples.