And you would be wrong, IMHO. Many people have untracked files or directories in their working directory ('cause they are working there) that they don't want deleted willy-nilly. Build files, modifications that should be on a different branch, etc. There's another thread active on the list complaining that git removes too much from the working tree.
** An SCM should be able to cope with both use cases. In fact I make it easy for the SCM by deleting EVERYTHING from the working directory, before calling git-checkout.
Most users of SCMs do make active modifications to the files in the SCM. It's not a system only for archiving static projects.
** Archiving static projects is not the ONLY thing I want to do with my SCM, it just happens to be the FIRST thing.