Another reason to use Mercurial (or Git or Bazaar)
A lot of the code I write for Thunderbird has to work on all the platforms it supports. Since I don’t own a Linux or Windows box, and don’t really want to waste the hard drive space on my MacBook Pro with a dual- or triple-boot setup, I’ve decided to install the other operating systems on virtual machines. (I’ve chosen VirtualBox, because it’s free, and I’m cheap. As an added bonus, it works quite well, too.)
If I was forced to use a centralized version control system, this would lead to a bunch of pain, since I wouldn’t want to check in a half-finished patch, but I would still really want to see what changes I made for one OS, to try and figure out what changes I need to make for the others. And I would want to be able to keep a record of what I did, step by step, so that I could undo stuff if it turned out to be a bad idea. (Hey, that sounds like a perfect task for a version control system! ;)
Since Mozilla uses Mercurial, I didn’t have to deal with any of that. I had my repository on the host machine, which I had cloned from the main repo, and I just cloned it into each of the virtual machines. Whenever I started a VM, I pulled the latest set of changes from the host machine; As I fixed stuff on that platform, I committed to the repository on the virtual machine; And before I shut down the VM, I pushed my changes back to the host machine. When I wanted to see which changes I made to get things working for the platform, it was easy, and propagating those changes to the other virtual machines was also easy.
When people talk about the advantages of distributed version control, a lot of the time they mention being able to still commit your changes when you’re on an airplane, and sharing in-progress changes with other people, but for people like me who do mainly self-contained stuff and don’t fly anywhere, supporting multi-platforms with virtual machines and still being able to track my changes might just be the killer feature.