Over the past year there were lots of talk about responsive vs. separate mobile websites. Of course duplicating everything, having two sets of URLs, having the risk to redirect from one site to the other are valid arguments against separate mobile sites. But to be honest, you have to admit that many responsive sites actually do not provide a perfect user experience. Even though their UI is adapted for use on smaller device it's not actually designed for it. You often see that these sites do provide a better display of information and functionality than non-responsive sites. But often they do present a readable but overcrowded UI.
So what now ? Is it all bad ? Well it just means that having two sites is not maintainable and having a single UI for all devices makes it difficult to optimize the user experience. But it doesn't mean you can't have two UIs based on common contents. Just some old Model-View... Of course, this is only feasible if you are able to define a model handling all extensions to your system. If you need to adapt your UIs for each extension of the site, this will basically have you maintain two sites. You need to actually be able to define a model and keep adding to the model without needing to change the view. That what you do when working with a blogging or a webshop platform.
Of course I still sometimes miss some functionality in such mobile sites which I would maybe be able to access if it were a responsive website. But again, even responsive websites sometimes need to hide some parts, require more scrolling to access it or actually put things at a place you wouldn't expect it. So I guess the main issue is especially to understand what's important for your user and what should be (easily) accessible on a mobile device. This of course costs money and time but will pay in the end.
Also I firmly believe that the purpose of users of different devices are often also different. When I'm on the go and want to access my analytics or adSense data, I actually expect to see a whole different UI than when I'm on my laptop and access them.
But it's not all black and white. Even if you have a mobile and a desktop UI, it doesn't mean that some parts of them cannot be shared using responsive UI modules/elements.