[jimwalsh]

It's pretty DRY as you'll only be writing media queries for the parts of the site that need to change. Many times you don't have to write MQ's for everything. Writing completely different sites like you mentioned earlier would be/has been a hassle to maintain as opposed to a site that was originally designed as responsive from the onset.

The use case that someone might come to your site on a tablet, laptop and/or phone is a pretty common use case nowadays. Designing and maintaining one site to cover all of those is the goal. Simple MQ's to shift to each one is much easier to maintain than an entire other codebase.