Why would I want to use Mercurial or any other DVCS client with a Subversion repository?
It lets us keep SVN as our central repository
Some team members prefer not to use a DVCS for whatever reason so it lets them carry on using SVN without interruption.
It allows me to work and commit changes (but not push!), search history and switch between branches completely disconnected. I can continue to work during network outages or while traveling when I don’t have connectivity.
You get full, fast history search.
Switching between branches is easy and fast.
Any automated processes which use SVN (i.e. automated builds and deployments) can continue to operate while everyone moves to DVCS.
It’s much easier to perform merges than regular SVN (via export/import patch queues – which I detail later)