I don't know about grazing in winter, but I have several reasons why I like fall / early winter calving. (My calving season is November through January.) Some of them may be specific to my region.
1. When the spring grass comes out, the calves are big enough to graze, and the cows milk production picks up. The calves really grow well in spring.
2. Calves are ready to wean about July or early August. I don't have any concrete proof of this, but it seems to me that the market often peaks about then, and starts to fall off a bit in September.
3. I'm weaning calves about the time it really gets hot and the grass starts to get short. That makes it easier on the cows and the pasture.
The only disadvantage that I'm aware of is that I have to take a little bit better care of my cows through the winter than I might otherwise need to so they'll cycle on time.
Someone else will have just as many good reasons to like spring calving.