I can't answer for ST, but as LDAP is an enterprise only feature, I would say it is only needed if you want to use LDAP auth within SmarterMail.

Disabling it/unbinding the port should have no effect on normal usage.

Think long and hard about running ANYTHING on a domain controller.  Running public services on a DC violates Best Practices.  If you install a service on a DC, and that service gets compromised, you could be putting your entire Directory Services architecture at risk, depending on the nature of the attack.

We won't even run an RODC that faces publicly, nor do we allow 3rd party vendors to authenticate (or otherwise) against our AD.  The risks far outweigh the rewards.