Ive never once had the light come on when my alternator has packed up, if the diodes in the rectifier go for the negative side, the battery light wont come on!

The only instance my battery light did come on was when the alternator belt snapped (which i was waiting for anyway, already had pre-warning by the weeks of squealing i had to put up with), and when the brushes went it also didnt go, (because it was still producing a voltage, enough to turn the light off but no where the rate it should of been).

All in all battery lights are crap, the primary function for them being that the alternator needs the light to commence charging (if bulb blows, it wont charge!!!!!)

You both have good points, volt meter gauges are USEFULL, but i also agree with you its not a critical gauge, if the alternator goes your not going to really damage the engine or anything (maybe top off a dying battery) but thats about all.