It does help but vets round here are seeing more colic cases than usual and reckon that horses are dehydrating. The river that supplies my boys' field has frozen over so I'm now shipping containers of water to them twice a day - and they were very thirsty the first couple of times I did this, despite licking snow.
My mare eats the snow, but i don't think they do it to re-hydrate themselves, as she is quite thirsty still, more so than usual but probably something to do with the ton of hay she has eaten recently!
My bloody shetland is dehydrated which i can only assume is because he doesn't like the cold water as he will drink the warm water I offer him. I say *bloody* as I caught him eating snow today.... so much for him not liking cold stuff ......