Respond to the statistics!
Smartphone and tablet "surfing" is steadily increasing, and closing in on 30% of total web traffic. Depending on your business type, this number could be more or less. However, it's always a good idea to serve up the best user experience no matter what device your visitors are using to access your site.
Responsive design does just that; it optimizes user experience on different screen sizes, aesthetically and functionally.
Have you ever visited a site on your phone and not been able to read the tiny text, or navigate to a different page? Did you find it user friendly?
Responsive design takes care those kind of issues, and has many additional benefits.