Mobile traffic to your site is predicted to increase to 50% in 2015. Even now, there are sites where it already reached 50%.
In six months, google will remove the option to target ads to desktop devices and exclude mobile devices.
If you buy ads on Google, you may soon be a mobile advertiser — whether you like it or not. And you could be paying for the privilege. (read more)
This means that if you are advertizing your Drupal sites on Google Adwords, you should better migrate them to a mobile, responsive theme soon. Otherwise you will be losing money since your site is not usable on mobile devices.
Drupal 8 will be responsive and mobile friendly out of the box:
Mobile: Drupal 8 will be responsive and work with HTML 5 and CSS 3
Until now, Drupal 7 included, mobile support has been limited in Drupal. There have been a few modules that were written to address this issue, but they are not easy to install and configure, and they are lacking the benefits of being a part of the core and playing well with the other components. Now, that’s going to change. (read more: Drupal 8 features)
But until then, there are Drupal 7 solutions which are good enough, like the bootstrap theme.