To make door openers and other kinds of devices, we need to be able to detect when someone bumps into or walks through them. Collision detection appears to only work for IsDynamic = true objects. I don't want a door to fall over when someone bumps into it just so I can detect the bump.
This ideally would work for objects with HasCollision = false ("phantom" in SL terms), too. A classic sensor type in SL involves invisible prims that detect collisions.