OK. I think I got it. Sorry, I was slow. So this is the case:

     - No initramfs, direct boot to the final rootfs (meaning disk 
drivers and filesystem built in the kernel),
     - /usr is on a partition per se,
     - the file-system of /usr is not the same as / (eg jffs2 for / and 
ext4 for /usr)
     - the file-system of /usr is not built-in (eg only jffs2 built-in).

     Then I agree that startup is impossible if the modules are under 
/usr. However, cumulating all these condidions is a corner case. And not 
all these conditions are imposed from outside; most of them are decided 
by the person who makes the install.

     Having /usr in a partiton different of / made sense in the time of 
unsafe filesystem, because there was more chance to corrupt /usr than /. 
But this is the past now. I've watched a hundredth of brutal power down 
on servers with reiserfs and btrfs filesystems, and zero filesystem 
corruption after I abandonned ext2 ~7 years ago. I confess I still use 
to mount /usr on a partition per se, but I think it has become just a 
habit which makes no sense anymore and I'm going to stop it.

      Didier