Here’s a use case for Darcs.
All the blogs hosted here run Drupal. I have a whole slew of add-on modules, themese, and a few patches. It’s a pain to manage them all, so after moving to Drupal 4.6.0, I decided to store my Drupal tree in Darcs.
Drupal 4.6.1 just came out. I used darcs_load_dirs to load it into my upstream repository, then darcs pull to pull it into my main tree.
It worked perfectly, first time. Very, very nice.
Now maybe in this particular case, Arch could have done as well, but I sure was impressed at how easy darcs made it all.