Responsive design > mobile sites
There are individuals and companies who still believe it would be in their best interest to maintain a 'desktop' version of their website, and a completely or mostly-separate 'mobile' version of their site, and this belief (especially in the corporate arena) was strengthened by a recent (2012) report by the Nielsen Norman Group, Mobile Site vs. Full Site, which recommended a separate mobile site with stripped-down features and different design. The idea of having a mobile-optimized design is good—but not with the cost of making it a stripped-down version of your 'full' site, as Nielsen seems to recommend.
There are many problems with having separate versions of the website, especially as we near a point where many sites are accessed more on mobile devices (tablets, smartphones), and less on traditional desktop computers: