My understanding of half-bridging according to http://www.cisco.com/en/US/docs/interfaces_modules/shared_port_adapters/configuration/6500series/76cfgatm.html#wp1307151 says, that an Ethernet segment with several hosts, bridged into an ATM PVC, the half-bridging feature enabled the router to work as default gateway for these hosts, even tough there might be several hosts on the segment.
This is something you can't do with regular RBE and you should use half-bridging.
However, I tried this feature some time ago on a 7604 with SIP-200 and an ATM-SPA running IOS 12.2(33)SRA. The result was that only one host could be active at any time, just like the RBE.
Could anyone clarify this for me and tell me if the half-bridging is working as it should (only support one host at any time) or there might be a bug in the IOS.