Hi,
Yes as such this is a "vCenter" level protection tool. As such it can't assist in the event of an ESX host failure
See below from the admin guide for a better definition can i could muster (http://www.vmware.com/pdf/vcenter-server-heartbeat-65-administration-guide.pdf)
"Architecture Overview vCenter Server Heartbeat uses an “Active–Passive” architecture which enables it to provide “High Availability (HA)” in a Local Area Network (LAN) or “Disaster Recovery (DR)” in a Wide Area Network (WAN) for vCenter Server, View Composer and SQL Server. The vCenter Server Heartbeat software is installed on an existing production server known as the “Primary” server running the protected applications (vCenter Server, View and SQL Server). An additional server, known as the “Secondary” server, operates as a ready standby server to provide service in the event of an application, system, or hardware failure, or when the Administrator needs to perform system maintenance. The terms Primary and Secondary refer to the “Identity” of each server instance, and these identities do not change" If you were looking for some DR-type solution for your hosts you could consider VSphere replication, its free if you an ESX5.1 license afaik (http://www.vmware.com/products/datacenter-virtualization/vsphere/replication)