Merging directories and initramfs are two different things. You can 
have one with or without the other. The great possibilities offered by 
initramfs have a counterpart: they are more painfull to develop, because 
you need to re-compress the initramfs and reboot for every try.

     The only thing initramfs provides here is /bin and /sbin before 
*any* disk partition is mounted - not only /usr. Note you can as well 
have a /usr in the initramfs (that's what I've done for my embeded 
Powerpcs). I don't know why they want to mount /usr while in initramfs; 
I don't see any advantage.

     Didier