That's not really a working copy, though. What some need is a tool that lets you check out a part of the repository as a working copy, without checking out the rest. By "part" we might mean more than one directory and its descendants (i.e. not a single root).
hmm, you could frankenstein together a bunch of trees to make it look like a partial checkout, but you couldn't make a new commit without all the parent tree objects up to the root. This sounds just like a subtree to be honest.
Are you frequently checking out a subdir of a repo and committing changes to it? Is it config?
The particular case I'm thinking of involves a repository which is large enough that full checkouts are slow, so you do partial checkouts to make things faster.
Maybe at some point there will be tools that let you do this with Git, maybe built on top of something like Git VFS. At the moment it just kind of sucks. Subversion and Perforce both handle it just fine.