My understanding is that eating snow is unwise in MOST survival situations, but NOT always.
To melt snow needs a lot of heat, and the eating of snow can lower the body temperature to a dangerous degree if it is already cold.

Eating snow is actually advisable if one is becoming too warm. Exertion when dressed for the cold can easily result in overheating and significant sweating, which is most undesirable since it may result in excessive chilling when resting, and also the sweat represents water lost.
Winter clothing should therefore be in adjustable layers.

If one becomes a little too warm when walking or otherwise exerting oneself, then eating a mouthful of snow produces a prompt cooling effect, arguably quicker than by adjusting clothing. Repeat each time you feel even slightly too warm.

I was told this by an army officer experienced in cold weather operations.