There is lots of confusion on the web about the benefits of 14-bit RAW capture. on the one hand people test it to hardly make any difference shooting test charts, on the other hand it is claimed that far more detail can be extracted from a 14-bit capture. One of the issues is that most of these tests use test charts or highly contrived circumstances, never a real situation. Therefore I decided to give you a real image. This is our garage in a very sunny day (it was Flag day when I shot this as you can see). I used default conversion in Lightroom of this 14-bit shot. In this sort of lighting, in order to not blow out the flag, you end up underexposing the garage. If you're smart in such a situation you might use strobes to light the inside of the garage. Another person would increase the "shadow fill" in Lightroom. Without those tricks, you cannot see the plethora of bikes and the fact that I really need to clean it out, which is perhaps a good thing.
The often repeated "truth" is that supposedly, with 14-bit capture, you get better detail in the shadows as you have the extra bits to play with. To test this, here are two details from the above guide image pushed 4 stops in Lightroom at 100%. I had to set the blacks to zero, otherwise the area stayed simply black. Guess which one is the 14 bits one!
Well? I'll tell you. The first one was shot at 14-bit. Surpsingly for an image pushed a full 4 stops, apart from being slightly brighter (even though the shutter and aperture are the same 1/500 at f5.6 and ISO 100) they are identical. You might even say that the 14-bit one has slightly less detail, but that is likely due to the slightly brighter appearance which makes the contrast and therefore apparent sharpness lower. To check whether this was due to maybe Lightroom not using the full 14-bits, I tried the same thing in Capture NX with the same result. Clearly, in real situations, using a Nikon D300, it is very doubtful that you will see any benefit from shooting at 14 bits. Not having tested other cameras with 14 bit capture such as the Canon 40D, I cannot tell you with certainty that this translates, but I would very highly doubt that you could see a significant benefit there. The reason of course is that the noise in the shadows is already large enough to be in the bottom bits in a 12-bit capture, so the only thing that happens is that you're imaging the noise with higher precision - not very useful and probably a waste of card space.