The default setting is 8-bit just because it requires a weak hardware to work.
Believe me if you guys tried to improve the scenes by adding several Video FXs, such as color corrector, 32-bit works more perfectly than 8 bit because the former one contains more data after calculation.
If you guys still do not believe, then try to import an MPEG clip and render it as WMV. Then you will see how 32 full range works more perfectly than the 8 bit!