Given that most mobiles (if not all) understand how to use Media Queries and a lot of legacy desktops do not, coupled with the fact that most traffic is still coming from desktops, as it stands there is nothing wrong with designing for desktop first for simple 1-2 step responsive sites. The only real problem with this is you have to account for 3g/mobile networks and find a away of serving your smallest images and media first.
I personally use mobile first because when you start taking into consideration height as well as the width it gets far too messy.