Smartphone penetration in the United States is staggering at 48%, up from 18% just two years ago. Among 25-34 year olds, 64% own a smartphone. If you don't design for mobile, you'll be left behind by your competitors.
One column design
Sidebars are often thrown onto a site without much thought. Often, the reasoning comes down to finding something (anything!) to fill the rest of formulaic 960 pixels horizontal space. Maybe it was through some misguided belief that flashy social media icons will rocket the site to the top of Alexa 1000. Tag clouds are cool, right?
If your site requires a sidebar to function, you're most likely doing something wrong. Either the content is not engaging enough or your navigation needs to be rethought completely. Only dense, database-type sites have a legitimate need for the extra level of navigation. In almost all cases, messy everything-including-the-kitchen-sink navigation sidebars only serves to distract the reader from the content.
If your users get bored reading content, or interacting with your site, you have some deep-seated issues that no reworking of the sidebar will ever fix. Providing a solution for your user's wandering eyes does not fix the main problem: poorly presented content. While a sidebar may take 25%-33% of a site's width, its actual importance is rarely as high as the space it consumes.
Designing for the limited real estate of mobile browsers forces you to focus on providing better content. Eschewing the customary sidebar in the beginning of the design process leaves you with no option but to create engaging content that's able to stand alone.
Efficiency, efficiency, efficiency
With a mobile site design complete, scaling up for larger screens is a breeze. Since most of the site's CSS is already defined in ems and percentages, changing font size and element width can instantaneously turn a one-column, tightly focused design into a glorious multi-column mess for larger screens. There's no question that media queries add a lot of complexity to your site's design. Designing mobile first is the only way to take advantage of the way media queries work: a single change in your CSS will be cascaded from 320x480 to 1024x768, 1440x900, and beyond.
Trying to force a desktop site's CSS into something that will work for mobile is a horrible experience. Desktop websites are messy creations which are never built with scaling in mind. Even if that new desktop-to-mobile conversion your team worked so hard on seems to display properly, updating anything in main site design will break the mobile site all over again. CSS is not well suited for this.
Most sane designers, when faced with a mobile design project for a desktop site, will ask to start from a blank slate. This creates its own set of problems. When this new project done, it's often ghettoed to http://m.yoursite.com, updated infrequently and only serving a portion of your site's content. Forcing your mobile users to bolted-on, attaché sites does nothing to improve their disposition to your product. Furthermore, any update you want to make to the site now requires twice the work: once for desktop, and again for mobile. It's not efficient.
If your ever envision your users viewing your site on a mobile browser, you should strongly consider going mobile first from the beginning. It'll save you a lot of future headaches.
The mobile experience
All innovation has been happening in the mobile space. The new iPad screen has near print quality resolution and it's a joy to behold. Quietly arriving at 2048px × 1536px, it blows traditional desktop and latop screens out of the proverbial water. The progress is shocking to think about when the vast majority of LCDs have been topped out at 1920x1080 for nearly a decade. Most users are stuck on less, since they're on stuck laptops and netbooks.
You can hold this screen in your hand, and the battery life is better than any laptop. Browsing the web on the iPad is an intimate, emotional experience, with direct one-to-one mapping of finger gestures to smooth, pixel-free interface. It's intuitive in a way that remotes, keyboards and mice will never be.
The bottom line
If you don't future-proof with site by designing with smartphones and tablets in mind, it'll cost you where it hurts. The mobile web experience is getting better every day, while desktops have been stagnating for over a decade.