I disagree. A compass can be very useful when you're lost. At least you know which way you're going on a particular road. Not too long ago I rented a Mercury Sable that just happen to have that compass in the mirror thingy, and it helped me in numerous situation. But maybe I'm just prone to getting lost...
Now I want one for my max as well (a compass, need not be in the mirror), but I haven't found one that's affordable enough. $29.99 seem to be the prevailing price, so this is not a particularly special price. $99? Maybe in Singaporean dollar or something...
BTW from my experience non-electronic (i.e. simple magnetic) compass are thoroughly useless. It'll just point at the large metal piece in front of the car, i.e. the engine. It'll just say North is straight ahead even if you run in circles.