I agree with not relying solely on LDAP. Requiring LDAP for the local address book imposes yet another installation/configuration/maintenance/support requirement on the RoundCube administrator. Also, for people who are hosting RoundCube on third-part web servers, LDAP may not even be an option.
Having said that, LDAP is a very common directory service provider for many companies and I think we need to support LDAP seamlessly. Many e-mail clients (Thunderbird, Apple Mail) support read-only access to LDAP directories as a supplement to their local address book. I like this model, which is generally where RoundCube is headed right now. There are some things we could do to more fully support LDAP directories, some of which are in Tickets already:
- address completion should incorporate results from an LDAP query (Ticket #1483899)
- authenticated bind to an LDAP server (there's a patch floating around somewhere to hard-code this)
- expanded query capability (useful for large directories). This should be kept in mind when adding fields to the local address book.