Medium sized orgs that want LDAP (and might not have the necessary Linux / Unix skills) just use Active Directory. Also, if you don't wrap it in Kerberos, it might be difficult to gain adoption. I say this as someone considering themselves a LDAP SME having setup multimaster openldap with 68 slaves globally using delta syncrepl Good luck!
Orgs do really expose their Active Directory server on the internet, to integrate with external servers ?
Most orgs put an OpenLDAP proxy in front of their AD server. AD has multiple known crash vulnerabilities in its protocol parser (fuzzing attacks can easily break it) and is too slow to handle the load generated from open internet access.