Interestingly, we asked a related question during Customer Development for our startup:
Will You Pay for a Responsive Redesign? - http://blog.roveb.com/post/15394363841/will-you-pay-for-a-re...
Will You Pay for a Responsive Redesign? - http://blog.roveb.com/post/15394363841/will-you-pay-for-a-re...
That being said, as the number of different mobiles devices quickly rises it will soon become near impossible to maintain a mobile site specific to every device.