I forgot to mention why it's nice to know whether a large portion of your mobile users are existing desktop users. It's good to know because existing desktop users tend to already be familiar with the interface, so that can skew the comparison results you get from Analytics.
If people already know where things are, they're going to know how to get to them quickly even on a small screen. They're more likely to be going to inner pages than someone who comes from say, a search engine or social networking site and isn't familiar with the interface if the interface is tough to use.
Now, if you find out a large portion of the mobile visitors are your existing user base, it's definitely worth polling your members before proceeding with your mobile version of the site. Find out exactly which elements, if any, are giving them a tough time, and focus on improving those elements rather than creating an entirely new layout that leaves them wondering where things are.