The idea behind using multiple, distinct partitions is that if , for some bizarre reason, whether it is human error, power loss, or an operating system gone wild, corruption is limited to one partition. In one typical case, power is lost while the system is writing to disk. This will almost certainly lead to a corrupted filesystem, which will be repaired by fsck during the next boot. Although fsck does it's best to make the repairs without creating additional damage during those repairs, it can be comforting to know that any such damage has been limited to one partition. In another typical case, the sysadmin makes a mistake during rescue operations, leading to erased or destroyed data.
Partitions can help limit the repercussions of the operator's errors.
With RAID-1, if a fault has occurred, and you can't get RAID to work, you can at least mount one of the two mirrors. Discover sysadmin mistakes now, and not during an actual crisis. Done carefully, this is not dangerous. There are several approaches that can be used. Alternately, use mkinitrd to build the ramdisk image, see below.