What do you think is the benefit of encrypting /boot? I can't imagine having anything sensible on there.
This would mean that if your laptop gets stolen, and you have a strong password and disable booting from other devices, the thief/fencer would need to find a way to reset the UEFI before they could install Windows on it and resell it. It's probably still possible, but it makes it much more difficult and they may just have to end up parting out the laptop and tossing the motherboard.
It's also annoying to have more than one system partition to deal with when all of the pieces of software and hardware involved are perfectly capable of working normally with a single encrypted partition.
Boot is a special case to some extent as most people aren't going to care if the few files in there are readable, but then again the initrd is in there, and that could contain things that would otherwise be encrypted if they were on root.
For example, some people set up dropbear sshd in the initrd, so it's possible (even likely, depending on the tutorial being followed) that it may have a real, unencrypted copy of the sshd host key(s) the main OS would be using, once it boots. Whether you care depends on the deployment, of course.
I suspect that in some obscure cases, allowing unknown parties to see the kernel binary may count as an info leak that could be used for other attacks, potentially even against other more secure systems that happen to be running the same kernel binary.