Then one can unify /var and /. What if one needs Postfix running in order to use SMTP to get a one-time password for the system to boot? Or maybe /var/spool is necessary for CUPS to print out boot logs on a remote printer?
Your argument is incredibly weak. If you need LDAP for your system to mount /usr then you definitely doing something wrong and you just should make sure that /usr is available before LDAP is needed. And uniting / and /usr won't give you ANYTHING, since in this case you won't even have the final / if LDAP is not available.
Besides, namespace pollution is a reason in itself to avoid throwing some extra stuff to /.