Ben,
HUMS is a subject of much debate. On the whole I'm of the opinion that we need it in the helicopter world, and that it does go some way to achieving the tasks you mention.
It is not however perfect. Sometimes failures occur which, or one reason or another were either not detected by HUMS or the human factor kicks in, and warnings are not spotted. It can also generate false alerts, meaning a fault is indicated, but in reality it is the sensor or instrumentation which is at fault.
As I said its far from perfect, but its a whole lot better than your first indication of a problem being the North Sea gushing in your cabin window
E-mail me if you want some detailed info.