Calling "git reset" on the command line (without further arguments) defaults to
--mixed, which is reason enough to make it the default for us, too.
But I also find myself using --mixed more often than --soft. The main use case
for me is that I made a bunch of WIP commits, and want to turn them into real
commits when I'm done hacking. I select the last commit before the WIP commits
and reset to it, leaving all changes of all those commits in the working
directory. Since I want to start staging things from there, I prefer those
modifications to be unstaged at that point, which is what --mixed does.