Well, there is always this scenario: when you can't even use CTRL+ALT+F2 to get to some type of terminal and only the power button, held in for ten seconds, will do. That's when you should not 'kill -9'.
I have heard the best practice advice for many years and I think that the 'you should send some friendly signal first' is not universally what works out best. For instance, if your Chrome browser is getting out of hand and the system is permanently doing some 96% wait for some reason, a gentle killing of Chrome will take ages and, when it restarts, you might get some but not all of your tabs back. With a killall -s 9 you can be back to work quickly with all your tabs (and underlying swappiness problem hopefully resolved).
I have heard the best practice advice for many years and I think that the 'you should send some friendly signal first' is not universally what works out best. For instance, if your Chrome browser is getting out of hand and the system is permanently doing some 96% wait for some reason, a gentle killing of Chrome will take ages and, when it restarts, you might get some but not all of your tabs back. With a killall -s 9 you can be back to work quickly with all your tabs (and underlying swappiness problem hopefully resolved).