Comment 8 for bug 872220

I don't think any such feature has been added. I certainly can't find it anywhere. Can you post a screen shot? What normally happens if you don't use the degraded boot argument is that the array will fail to activate. If you have filesystems that can't be mounted at boot, the system asks if you want to continue booting or not, but if your root filesystem is the one that can't be accessed, then you can't actually continue. Is your /etc/fstab pointing to the filesystem by its UUID or the /dev/mapper/xxx device? If it is the raw device, that would explain the system telling you it can't be mounted and not being able to continue.

Just to make sure, I reproduced a mirrored setup in a VM and booted without one of the disks and the system booted right up off the underlying regular disk, bypassing the dmraid since it failed to activate, and the root fs UUID could be seen on the normal disk.