As chscag states above - it is the migration assistant that is being used on first boot - only you won't know it as that - it's just a part of the set up program on a new Mac. (And actually, this capability is a part of the Setup Assistant of a clean install of OS X.)
Those that get a new Mac, pull it out of the box, can't wait for another hour or so to use it and go ahead and set up a user account so they can use it "now" - they would have to at some point later, manually open Migration Assistant to bring over their data.
Migration Assistant will not delete existing user accounts already set up on the machine. Which means that if they've been using that new Mac for some time, they'll likely have to deal with the hassles that can (will) be associated with then moving data to the user account they plan on keeping.
Not sure I would call it "leave the Mavericks OS alone" - it would be more analogous to doing an upgrade installation of the OS - when it's done - all your stuff is still there right where you left it, but it has a new OS. In the op's case - since both will be running the same OS, when done, the new computer should mirror the old, right down to the user accounts, login, apps, etc..