I think that will be usefull if an app could make some changes for vibration:
- if the device is in the pocket (using the proximity sensor and, maybe, the light sensor) it vibrates strong
- if is in the desk (in my office, for example), vibrates but not so much
- if completely dark (and the proximity sensor says that isn't anything over it), don't vibrate (for example, in the night while sleeping)

Everything configurable, including the vibration level. Only while receiving a call, of course.