About digital camera exposure index settings (a.k.a. ISO speeds, though they are not at all the same thing as the ISO speed rating of a film): there is no point applying different analog gain to different photographs once a single choice of gain achieves two things:
a. full well signals are converted to a voltage that is within what the subsequent processing can handle without clipping
b. the dark noise level in the signal is amplified to a level comfortably above all subsequent noise sources, including quantization error in A/D conversion, so that the amplified signal is strong enough to not be significantly affected by downstream noise sources.
Both conditions are are probably met by a MFDB with a good 16-bit or even a true 14-bit A/D convertor, because the gain amplification is done just before A/D conversion, so that the A/D convertor is the only downstream noise source to worry about, and the DR of such an A/D convertor should be far greater than the DR of the signal being converted. Amplifying a low light (high exposure index) signal more would simply risk blowing out highlights while not improving noise levels in any significant way.
In other words, for MFDB's with adjusting of final output levels in the digital domain ("fake") might be as good as or better than adjusting them in the analog domain ("real") when dealing with high expsoure index shooting; that is, in limited light situations where the sensor has to be given less than full exposure, so that no photosites get close to full well signals.
Variable gain is far more useful in CMOS sensors, and when using lesser A/D convertors, like 12-bit and maybe even lower quality 14-bit. It is useful with CMOS because the variable gain can be applied far earlier in the signal path, such as at the bottom of each column of pixels, so that there are more noise sources downstream of the amplification, like noise arising in transportation across the sensor. Emil Martin has shown us good evidence that this noise down-stream of variable analog gain is the dominant noise source with some Canon DSLR's at low to moderate gain levels (low to moderate ISO speed settings.)
