Bit depth is about dynamic range, not the number of colors you get to capture: Digital Photography Review

Trigger warning: techie stuff.

"...the difference between 12 and 14-bit Raw has less to do with subtle transitions (after all, even in the example I describe, the tones around middle brightness would be encoded using 256 levels: the same number of steps used for the entire dynamic range of the image if saved as a JPEG or viewed on most, 8-bit monitors). Instead it has much more to do with having enough Raw values left to encode shadow detail."
"So, even though you started with a camera whose DR outstrips its bit depth, that stops being true as soon as you hike up the ISO: instead you just go back to encoding shadow noise with tremendous precision."