My understanding is that the 14 bit A/D gives a finer quantisation of brightness levels rather than a greater range between the darkest and brightest parts of an image which can be accurately recorded. Christian Buil's 40D Test
has some interesting comments on the subject.
I confess that when the 40D specs first came out I made the mistake of assuming the 14 bit A/D would allow for greater range rather than finer quantisation. I think there are real benefits to 14 bit A/D, however, even if with no significant increase in range. Quite apart from the very real benefits in photometry referred to by Buil, that finer quantisation would allow for more extreme intensity stretching in post-processing with fewer artefacts if one needed to highlight very small variations in intensity. A classic example, IMO, would be the lace and decorative stitching in a wedding dress (provided the dress wasn't overexposed in the first place).