The floatingpoint pixel pipelines are required for Full DirectX9. And it is likely that Doom III will support 64 and/or 128 bit framebuffer when it comes out. (John Carmack is one of the big names who want more precision for calculations)
The 128 bits are for the framebuffer not for the monitor. Monitor will show those images dithered to 8 or 10 bits/channel.
this for the simple reason that monitors cannot produce the additional dynamic range needed to display it.
BUT. the additional bits are needed quite badly in framebuffer especialy now when games are using more texture passes to form the final pixel on screen. Now when you have enough lights with per-pixel fx or many transparent layers you will see banding and color errors.
This is bad, but it is also inpossible to do some subtle fx without enough accuracy of calculations. (IE. Fog and some fx in Final Fantasy movie used something like >4000 very transparent layers)
to see banding check this little demo and set brightness of stars low.
http://planet3d.demonews.com/PWGravity3D.htm