eckhard:
The reason is that most auto systems that I've seen would have trouble interpreting the electronic signals from the beam bar deviation and supplying a sensible correction if the aircraft heading was more than 30 or 40 degrees from the desired course. The systems can cope with a bit of drift but 180 degrees may be asking a bit much.
I believe you're thinking in terms of a localizer, where the course window (aka, OBS) only references a HSI to the correct position, but absent that the localizer deviation will still be correct, albeit offset and confusing. With VOR, though, the actual track deviation will be in error with an incorrect value in the OBS. (The OBS may be a numerical value window, or simply a movable arrow in the "cheaper" HSIs.)