The point of switching to 16 bit is not making it real 16 bits, but making it more robust against strong pp.
Download this 8-bit TIFF file: 8bitposterization.tif, with 2 curves applied to a smooth gradient. If done in 16-bit mode the image remains unaltered, but if we stay in 8-bit mode a clear posterization will appear because of the 8-bit rounding caused by the first curve:
Switching to more than the original 8 bits will not improve the effective bitdepth of any resulting image, but will prevent posterization as in the extreme example above.
A very nice demonstration, but we must remember that images taken with a real camera have noise, and the dithering produced by the noise will reduce posterization. Here is your 8 bit image with 2% noise applied in Photoshop using the Gaussian noise function.
8 bits per channel is usually sufficient for rendered reflection prints with a contrast ratio of up to 288:1 (8.2 stops) as in the ICC PRM
. This assumes that you got everything right with the camera exposure. Prints will be with us for a long time, but as higher dynamic range display methods become more common, we will need more bit depth.
has calculated that 8 bit sRGB has a DR of 1.6 orders of magnitude (5.31 stops), allowing for a maximum of a 5% difference at the low (shadow) end, which is quite generous, since the average person can detect even smaller changes. Of course, this discussion applies only to luminance, and when color images are involved, one needs even more bit depth as when going from sRGB to ProPhotoRGB.
From a scientific and human perceptual viewpoint, the use 16 bpc makes sense, and one might as well use a wide gamut space such as ProPhoto to make use of the colors captured by the camera.