A number of years ago we sent my gelding to my BIL's farm in Saskatchewan where he was winter pastured ( I thought he was being fed in the yard). When we got him back in the spring, he was very underweight, and I was heartbroken. He may have done OK if he was accustomed to foraging or if there was supplementary hay available, but neither was the case. After this experience, I would be very hesitant to agree that winter foraging is enough for a domesticated horse. I think most horses have become accustomed to being fed regularly. Wild horses do survive on grazing alone, but their body does not maintain the condition we expect of their domesticated counterparts.
That said, grazing is good for them as it encourages them to stay active, it keeps their teeth in better condition and wears their hooves a bit. In my opinion however, it is not an adequate substitute for regular feeding, floating and farrier work, especially in horses that are worked regularly. The quality and amount of forage must also be monitored carefully, with availability of good forage changing with every snowfall / freeze -thaw cycle. I am also a bit skeptical that horses get enough moisture from only eating snow.