'Maintaining two sites' is the key word here. If you have a lot of maintenance, that would really be a pain in the butt.
I will also, eventually, when I get time, convert my sites to responsive. Google is rewarding it, and it is also better, when done right, for mobile users to see, say, if there are columns on the pages, to view each column covering the whole screen width and the next one below that, instead of all the columns squashed together as it would be if you open a non-responsive site on a phone.
There are cases where a redirect to a separate mobile site would be better. Newspaper sites, to mention an example, are usually very complex and would hardly fit into a grid or @media solution. Also, if the design of a site has been tailored to a specific width.
When I make new sites now, I do my best to keep them responsive, although I sometimes have some validation problems.
My tuppence...