I hear what you're saying about the grains to feed animals in the winter too. We feed our chickens organic layer feed, and they go through a lot of it during the time when they are producing no eggs. However, they also eat all of our household scraps, which I think is much better than composting them. They produce the manure which goes into next year's garden.
That being said, from what I've seen, eggs are definitely not an environmentally friendly food. Especially in the months when there is no significant forage for the chickens. But personally, I would never butcher our chickens just because they needed to eat over the winter. Just like I would not butcher the dog because he needs to eat. They are our friends as well giving us food. I respect them for their sweet chickenyness. Probably if we didn't have children, we wouldn't have chickens, but it is important to my daughter to raise chickens, and I've grown to appreciate them too.
I think another interesting question to ask is this: Should the top priority of animal agriculture be sustainability and efficiency? Or are there more important ethical considerations when we are using animals for food?