The case for the /usr merge

The cases where there is a shared /usr implies a degree of sophisticated system administration already, probably capable of rolling out more complex changes if necessary when there is a platform update. That said, the common case is going to be patch deployment that should not require changes outside of /usr and for admins used to RHEL they won't expect intra platform upgrades anyway.

Actually thinking of RHEL I see a use case for virtualized servers where many OS instances share /usr but have private /var, /etc and /opt. That could provide great gains in memory and disk dedup for "free"