In ACR, why is the Brightness set at +50 and Contrast set at +25 by default?
B&C are secondary adjustments to Exposure, Blacks, Recovery and Fill and would be adjusted last if at all. Would it be logical to set new defaults for B&C to be zero when processing RAW files?
Carrie, the numbers are essentially meaningless. As Jeff said, they're just defaults. If you find these initial B and C values are not to your taste, you can twiddle with them till you come up with values that you find better, then save them as your new defaults (or make it a preset).