This has been broken since 6.4.0, where it works fine. After the current changes on 6.4, the following scenario produces an invalid configuration.

Steps to reproduce:

1. Go to Users->Authentication;
2. Go to the LDAP settings tab;
3. Enable LDAP authentication, add an LDAP server and fill out the mandatory fields;
4. Press [Update] to save the configuration (OK so far);
5. Go to the LDAP settings tab again;
6. Press "Remove" link next to the server that was added;
7. Press [Update].

Result:
The configuration is saved, the LDAP is enabled, but there are no servers: 

Expected:
There is an error and the user is prevented from saving the configuration (screenshot from 6.4.0):