Of course, creating a responsive site is more difficult than creating nonresponsive sites. We're designing more than one look for the site: a sidebar floating right for users on a desktop might slide to the bottom of the page for mobile users; a nav element laid out horizontally on large screens might stack vertically for small screens. We have to develop CSS stylesheets with media queries to render the different designs, with percentage-based (rather than pixel-based) widths, and with appropriate rules to shrink down images and other fixed-width elements.
So why go to the trouble? We all know that more and more users now access websites via mobile devices - and the numbers will only grow as time moves on. Recent studies such as one from Smart Insights.com highlight the business case for taking the time to present users with mobile-optimized sites: phone-friendly sites generate more traffic, better user engagement, and more money.