Here's another take: http://www.second-opinions.co.uk/vegetarian.html#link1
One missing piece regarding the idea of sustainability of ruminant farming versus grain farming is that the former tends to rejuvenate the soil, and can be performed in areas unsuitable for crop growth. The latter, on the other hand (unless performed in concert with ruminant farming), encourages deforestation (to locate healthy, nutrient dense soil), using up the nutrients in the soil, maintaining the soil via petroleum fertilizers, and then abandoning the land once depleted/eroded.
Ruminants are a key piece to the puzzle in sustaining the fertility of the soil when growing crops.