If only this were true. But it's not: if I work on a project where my colleagues keep the source in git, then I have to use git (whereas if they use vi, I am still at liberty to use emacs). That's the problem with CVSs -- they have a lot more inertia than most other tools because they are, by nature, shared by groups.
I fear that Hg-Git would be one of those things that is an absolute delight for as long as it Just Works, then abruptly transitions into an absolute horror the moment something goes wrong.
I don't use Hg-Git, but I do use git-svn. Just like Hg-Git, I'm sure there are some caveats, but what finally got me to switch is that (at least for git-svn) it's a real live Git repo. Which means the code I write, my actual work, is all still there. The fix to my nightmare scenario? I do a fresh svn checkout to a new directory, and I do git checkout, I copy the files out, I do an svn commit of that version of files.
Definitely far from ideal, to be sure, but in my mind, knowing that was the worse case scenario, put me quite at ease. (Sure I'd read about git svn dcommit, but knowing that did set my mind at ease.)
The term you're looking for is "network effect". http://en.wikipedia.org/wiki/Network_effect