Livestock and Deserts

Questions of whether or not we should all become vegetarian because the conversion rate of grain to meat is 10:1 ignore a lot in order to come to that conclusion. Sure cattle are terrible at converting grain to meat--but they're not really supposed to be converting grain to meat.
Any animal converting grain to meat is a worst-case scenario. And cattle are the worst of all. Pigs do much better at 5:1, and chickens are just over 2:1. But one role of animals--particularly ruminants, is to bring otherwise un-farmable land into the food chain. Pigs excel at bringing food waste back into the food chain, and have long been raised on slops--otherwise unusable food. Pigs have even been used to manage human sewage and bring it back into the food chain--although that's not something I would recommend. And chickens are champion foragers. Given a bit of woodland and some pasture, and they do quite nicely at feeding themselves most of the year.
But most important is the role of animals, primarily ruminants, in building soil. People like Joel Salutin at Polyface Farm have found how to maximize this aspect of animal husbandry. Intensive rotational grazing by cattle followed some days later by rotationally-grazed chickens increase the speed of soil production, add nitrogen to the soil, and provides insect control.
Sustainable farming pretty much requires animals as part of a land management plan. They are just too darned efficient, and offer compact calories, access to nutrients that are difficult to acquire any other way, and the opportunity for increased income. Sustainability, after all, includes economic sustainability.
Alan Savory did this TED talk about the role of ruminants in wild systems. It's worth a watch.