Git is certainly much more widely used in general for open-source, but there are plenty of major projects that are on Mercurial. Python, Vim, Mozilla, dovecot, Go, NetBeans, and mutt all come to mind really quickly. Something like Kiln Harmony means that you, as a Git user, no longer need to care that these projects are on Mercurial. And conversely, if they want to use a project that's in Git, they can, from the security of their Mercurial workflow.