The way I understand the importance of the catch-all log is to be able to debug the system that fails to boot. It is writing a log in /run even before a real filesystem is mounted. At this point the system should give access to this file even if root is not mounted as rw yet. There may be many ways to have this copied to a root fs, which means it is already mounted rw, and by this time it seems as the system doesn't have a problem booting, so this log is of little use to getting it to boot.
In 66 this is written as /run/66/log/0/current, and I have user/root copy it after a shell is executed, this helps when I would change something and it is not booting right on the next boot, I can compare the before after. Part of 66 booting procedure is to activate tty12 as early as possible, instead of the insecure sulogin that appears in runit. From tty12 you can read that log and mount things manually and change/fix what is wrong. You can also decrease/increase verbosity in that log. tty12 is a special setup in 66 where root can't login, only a user can, a security measure I find as a great idea. If tty12 isn't accessible, that means the uncaught-log isn't, and I don't think there is anything in the system that could have copied that log into something else. So, if you can have it copied and saved is usually when you don't need it, but when you do need it it can't be copied. I hope this makes sense to someone other than me :)