While Google prefers sites to be responsive, there are advantages to custom designing a mobile site. Most visitors aren't looking for half the information on your desktop site when they're searching for stuff on their smartphones. They want deals, coupons, menus and directions - and a call me or email me feature at the touch of their fingertips. I've used Google's mobile friendly link a number of times where the site is clearly not responsive, and they still gave it a mobile friendly status.