why do shmem attach?

Exec-on-startup was removed by Bruce long time ago.
Why we still attach to shmem after fork?
Or shmem inheritance is not portable?
Also, all this ShmemIndex stuff seems to be useless
(except of backend PID lookup but it's for sure
should be in separate hash table).
And why separate shmem segment (!!!) is used for 
Slocks (ipc.c:CreateAndInitSLockMemory(), etc) - they
use so small amount of memory!

Just wondering...
I'm going to use old shmem init code for WAL but would like
to denote that shmem stuff need in cleanup.

Vadim