As explained in the patch, the Bang-bang thermal governor is intended to control simple cooling fans that can only be on or off but not scale to a given load or boast other functionality. "The bang-bang thermal governor uses a hysteresis to switch abruptly on or off a cooling device. It is intended to control fans, which can not be throttled but just switched on or off. Bang-bang cannot be set as default governor as it is intended for special devices only. For those special devices the driver needs to explicitely request it."
The Bang-bang thermal governor remains under discussion on the kernel mailing list after patches for it originally appeared a few months back. Bang-bang will hopefully be ready for an upcoming kernel release (Linux 3.17?) and the latest technical discussion about it can be found via the LKML archives.
One Linux kernel driver already planning to utilize the Bang-bang thermal governor is the "Acerhdf" driver that serves as the fan driver for Acer's Aspire One and other Acer systems where it has a simple fan that only supports being on or off. Up to now the acerhdf driver has handled its own on-off controls by post-manipulating the kernel's thermal subsystem trip point handling but will now be able to utilize the unified Bang-bang governor.