The HCW is needed when you want to configure or maintain your Exchange 2013 Hybrid configuration. When checking the prerequisites, the Exchange 2013 HCW may throw the following error message:
Subtask CheckPrereqs execution failed: Check Tenant Prerequisites Deserialization fails due to one SerializationException: Microsoft.Exchange.Compliance.Serialization.Formatters.BlockedTypeException: The type to be (de)serialized is not allowed: Microsoft.Exchange.Data.Directory.DirectoryBackendType
The issue has been documented in KB2988229. An Interim Update is available, as reported here. The IU is available for Exhange 2013 Service Pack 1 (CU4) and Cumulative Update 5. Unfortunately, the IU is not available publicly, but must be requested through support.
The fix will be incorporated in Exchange 2013 Cumulative Update 6.
If you must, you can use Exchange fellow Steve Goodman’s instructions documented here, which describes the process to manually configure Exchange 2013 Hybrid deployments. Be advised that, as Steve also points out, the Exchange Hybrid deployment support status depends on the ability to run HCW successfully.