According to a Google study, a mobile-friendly site turn visitors into customers.  Check out the article posted on the Google Mobile Ad Blog.

 While nearly 75% of users prefer a mobile-friendly site, 96% of consumers say they’ve encountered sites that were clearly not designed for mobile devices. This is both a big problem and a big opportunity for companies seeking to engage with mobile users.

-What Users Want Most From Mobile Sites Today, Google Study

Source:

http://googlemobileads.blogspot.com/2012/09/mobile-friendly-sites-turn-visitors.html