pretty sure any developer is likely to have several of these installed; I know I've needed to install them to pull stuff from various open source repositories, regardless of the fact I use git for personal projects

Indeed. E.g., I am using git for more projects than I use mercurial. To these projects, however, I just submitted a limited number of change sets. Most heavily I am using mercurial instead, which I also would choose for future projects. Maybe it would more accurate to measure the number of submissions per time interval.