Does secure boot matter if you never turn the computer off?
Security benefits are questionable. There is no scenario where non-technical users boot into a malignant OS. They open mails with strange attachments. The security analysis leading to this initiative is some kind of fantasy novel.
According to the report there is, and I believe the report is correct, when the "malignant OS" is not actually a full OS, but rather a rootkit. This rootkit might have been the result of a non-technical user opening a mail with a strange attachment.
It's supposed to protect from a very theoretical risk.
I'm running Arch on a "professional" HP ProBook with secure boot activated. This laptop has no official linux support whatsoever from HP. It's fairly recent, too – 2018 I think.
The bios allowed me set up my own keys which I use to sign the kernel. A fun fact is that with the way it is setup currently, I refuses to boot windows because of the... wrong signature!
Not all manufacturers allow this, and even then, it adds considerable user complexity to have to do this to install Linux, as opposed to a normal liveUSB GUI flow, not to mention that it screws up dual-boot, as you said. So, it is practically a requirement, even if it is technically possible to work around it with good technical skills on specialized hardware.
I think in this case there are two problems, more related to the hardware and SecureBoot implementation rather than secureboot itself.
1. Shoddy hardware that doesn't allow the user to control it.
2. A process that is relatively involved, although I'm not sure how you could go about providing an "easy way" for people without technical skills.
One way or another, the keys have to get in the UEFI. It's technically possible to configure them from inside Linux (while it's running) – this worked on an HP EliteDesk – but the UEFI SecureBoot has to be disabled. I suppose that's relatively simple to do, certainly simple enough for someone interested in trying linux (as opposed to people who don't care what os they use as long as they can accomplish what they want with their computers).
I guess a nice way would be for the UEFI to expose some sort of interface where the OS can change the keys and then, on reboot, asks the user if they really wanted to the change. But this would still be a problem for the initial setup, because, if booting linux (instead of the usual windows) you are actually booting a different OS than what the UEFI expects.