If you lie down on your left or right side and have your head on the pillow you look to the device normal while the accelerometer thinks you're looking in a 90 degrees angle. It doesn't know its owner is lying in this position. Its an exception really, and justifies the ability to put off the accelerometer like you can put off a GPS or BlueTooth.
Because being able to change your mode from landscape to portrait allows you to use your screen better in some circumstances.
Save extra $400? Then don't buy any mobile device... it just doesn't make sense to compare a $200 entry level touchscreen phone to a $600 flagship product.