The main reason we need to worry about responsive e-mails is simple: about 65 percent of e-mails are opened on mobile devices (smartphones and tablets). The remaining 35 percent of e-mails are opened on desktop. Additionally, responsive e-mails have more engagement than nonresponsive ones.
In addition to this, e-mails opened in desktop have more engagement than e-mails opened on mobile devices.
Check out the following articles:
Nearly 65% of e-mails in the U.S. are opened on mobile devices: https://www.internetretailer.com/2014/01/23/nearly-65-e-mails-us-are-opened-mobile-devices
65% of marketing e-mails were opened on a mobile device last quarter; Android tablet use doubles: http://www.phonearena.com/news/65-of-marketing-emails-were-opened-on-a-mobile-device-last-quarter-Android-tablet-use-doubles_id51864