Since nobody can post even one example of someone being dropped by a no hands belay on a properly threaded gri gri (other than slow motion) I will tell you why Petzel states that you should not let go of the brake hand in two words: fucking lawyers.
First off, 12 hours isnít really long enough to let people dig up examples...
Second, I highly doubt it has anything to do with lawyers (paranoid much?).
Third, it has already been stated why it makes no sense to let go of the brake. But to re-iterate;
1) If there is high friction in the system the cam may not engage if there isnít some resistance on the back end, aka the BRAKE HAND.
2) Itís a bad habit to get into since most people expect to use many different devices in their climbing...
Let me pose your question a little differently...
Can anyone give me a single example of a climber being dropped by the belayer while firmly holding the brake strand
on a properly threaded gri-gri?
It should become instinct when catching a fall with any device to let go with the none-break-hand. Even with an ATC, I like to see the ďoff-handĒ travel over to the brake side of the device... This practice is safer than simply throwing your hands up in the air to let the camming action do itís thing.
And one last time, resistance on the brake side of the device aids in the cam engaging... that is why Petzl (and everyone but you) keeps a brake hand on the rope when catching with the GriGri.