The trade off, is, well, now you need a dedicated mobile site. Subsequent changes to your codebase suddenly require twice the effort, since you're changing two different sites.
No, every change to a responsive site can break other resolutions/media queries without your knowing. Or do you routinely view your site on an iphone, Android, ipad, Kindle, netbook, laptop, and 30" monitor?
You're claiming responsive comes for free, or is no harder than purely non-responsive. Definitely not true.
