>Which is a very sane default for most people
I couldn't disagree more. Having an open mic setup for gaming is both insane and completely inconsiderate to everyone else in the channel/group.
It's hard not to personally judge people that join a group, whether that's a friend of a friend, someone looking to try out, public LFG lobbies, etc and you can immediately tell they have an open mic configuration. It's maddening.
Breathing, keyboard smashing, mouse clicking, background noise, eating, or exasperated whining every time you die/lose are all annoyances you push on the rest of the channel/group with an open mic configuration. At best it's an annoyance. At worst it's actively disrupting people's game play and team communication.
For the love of all that is decent with humanity, use push to talk.