It's never been a great argument. Even non-atomic Linux distros have you reboot after updates. It's just the safest way to ensure that everything is running with the updated packages. You're kind of in an untested state if you have mixed versions of applications and their dependencies running.

Plus, updates aren't the only thing that require reboots. Various config changes will need a reboot or at least require you to log out and back in. Even just adding your user to a group needs you to end your session for the change to apply.

That depend on your needs.. linux can do live update if you need that, usefull on servers..

But atomic versions as more target to desktops or containers where you need to have know working setup and when you upgrade you replace it by newer one..

So you dont update per se... You install the new version in a separated partition and boot into it the next time you restart.. Same with containers, you just destroy it and recreate with the new version..

If the new version fail you boot back to the old original version that is know to be working and have not being replaced..

The ideia is to ensure a known to be working system is always available..

Wait until Linux can run itself from the top while keeping your work intact.

See https://lwn.net/Articles/1033364/