Well I don't see a reason why it would not be technically possible to have an "as input" setting for bit-depth - but having such an option would introduce a lot of usability issues.
And I do se a reason to have it: because I need it
If you
need a 16-bit output, just set the output to 16-bit manually.
and because the standard red boock is 16/44.1
If your argument is that the output should be 16-bit because CD (Redbook audio) is 16/44.1, then you don't understand that it doesn't matter whether Media Center outputs 16-bit, 24-bit, 32-bit etc.
As long as the output bit-depth is equal to or greater than the input bit-depth, it doesn't make a difference to playback.
However, it does make a difference when you have a playlist that mixes 16-bit and 24-bit files for example, as that will cause the DAC to switch modes, which can prevent playback from being seamless. (many DACs will have delays or pops & clicks when switching output modes)
Or if you were to play a 16-bit file at 100% volume and then reduce the volume, it would have to switch to 24-bit (or the highest supported by your DAC) which would definitely interrupt playback.