According to Google:
"By 2013 more people will use their mobile phones than PC's to get online"
"Mobile searches have grown 4X since 2010"
"There will be one mobile device for every person on earth by 2015"

Users expect their mobile experience to be as good as their desktop experience. A bad mobile experience can cost you customers.

"57% would not recommend a business with a bad mobile site"
"40% have turned to a competitors site afte a bad mobile experience"
"23% of adults have cursed at their phone when a site doesn't work"

Customers are connecting with the businesses in their local area on mobile.

"95% of smartphone users have searched for local information"
"61% of users call a business after searching and 59% visit the location"
"90% of these people act withing 24 hours"

Apps are fun and useful, but many prefer mobile sites for shopping.

"81% of users prefer mobile sites to apps for researching prices"
"79% prefer mobile sites for product reviews"
"63% prefer mobile sites for purchasing"

Businesses like yours can drive purchases with a mobile-friendly site.

"70% of mobile users have compared product prices using their cell phone"
"65% have read reviews on their phones"
"50% of mobile searches lead to a purchase"

Now that we have seen what Google has to say about why a mobile website is necessary, do you think google will start giving search result preference to mobile optimized websites?

Sources: Lightspeed Research, 2010; Google "The Mobile Movement: Understanding Smartphone Users," 2011