It would be useful to be able to tell if an object implements Combat 2 damage events via llGetObjectDetails. It's possible to discover this using llDetectedType in events that support llDetected* functions, but other events have no good way to get this info.
The use case here is something like a raycast-based weapon. These typically execute out of a control or listen event and use a raycast call to detect objects, not a sensor or collision that would have access to llDetectedType. Being able to tell if the object is damageable through llDamage allows for smartly delivering damage this way vs. other methods (e.g. channel message-based armor systems like LBA).
OBJECT_DAMAGEABLE would be a simple boolean just like OBJECT_PHYSICS and would return TRUE for the same cases that llDetectedType() & DAMAGEABLE does.