The reverse lookup zone was originally used for network troubleshooting (ping, traceroute, dig, etc). For external DNS (especially mail servers) reverse lookups are essential. Most ISP/Hosting services rely on reverse lookups for anti-spam techniques.
Active Directory does not require reverse lookups but without them clients may not be able to fully resolve host names. Some will argue that adding reverse lookups (PTR records) to an Active Directory Integrated zone will slow your DNS query’s. I have not found this to be true.