Mobile sites were originally conceived for very limited devices with extremely throttled download speeds. They are usually* hideous affairs with barely-functional drop-down lists, that leave out any branding or style that may be present on the main site. Most of the time there is absolutely no useful information on the front page, meaning that a user has to click-through before there is any arresting content.
Nowadays, there is very little gap between the net speeds of desktop and mobile devices. The only reason I can see to produce a mobile version of the site is to compensate for the smaller screens, and even then, that issue can be mostly avoided by good, clean, reflowable, uncluttered site design.
For myself, I always view mobile versions of a site as a nuisance, and click through to the desktop version as soon as possible.
With that said, are there any outstanding reasons to produce a mobile version of a site that is substantially different than the desktop version?
*I know better than to make universal statements, but this seems to be the rule, not the exception.