You might be right that setting ISO of 100 on the D100 would result in a "noisy" image, and that was the reason for disabling it. (any linkage?) But I would expect it to be something more than that, or for the difference to be so severe that it was completely unacceptable. Even if ISO100 produces images which as noisy as ISO400, it's still not a horrible idea to include the mode?
There were some discussions and speculations during the D100 launch quite some years ago, but the gist of it is that the sensor is probably so insensitive at ISO100 that the signal cannot be picked up accordingly. Err... I suppose something like using a lousy mic against a good low bass singer, where the mic can't even pick up any of the low notes, such that no matter how much the sound engineer ups the amp, you can't hear the bass simply because the mic didn't 'hear' it.
I suppose the discussion on D100 and ISO should end here, it is F30 vs F31fd after all. I'll say, "Pay less for F30, value for money" (hey, it rhymes!).