Ok, I've created an enhancement request. Please vote for it
OK I voted!
Storing contacts in LDAP is probably a non-starter.
In large deployments, you'd absolutely overwhelm your LDAP master's write throughput and bring the whole system to a crashing halt. Also, it's probably a bad idea from a server framework standpoint to attempt to push all the contact-related database changes to LDAP, and it's definitely a bad idea to have part of your mailbox contents in the database and part in LDAP. Plus, having a parallel ACL system for LDAP address books isn't likely to happen any time soon.
Actually, it might be possible for the ZCS server to export an LDAP interface to the database alongside the SOAP and REST interfaces. Might be worth considering that.
I voted too