Growing crops to the exclusion of animal husbandry increases demand for chemical inputs. Those chemicals are not only polluting but are hard-won only by the massive expenditure of fossil fuel energy. Additionally, you'd be removing a vital contributor to soil structure, animal manures. That's particularly vital in third-world countries, where subsistence farmers can't afford to buy chemical fertilisers in any case and where seasonal drought is a big factor in the erosion of soils which lack good structure (dust bowls, anyone?) Then there's irrigation. Look up how many gallons of water it takes to produce each nut on an almond tree in California (I'll save you the bother: it's around ten). In certain parts of Chile environmental devastation is going on in areas where avocados are grown for export because so much water has to be diverted in order to water the trees. You can't grow useful crops on thin, dry soils on steep rocky slopes or at high altitude, but you can rear animals which you can move around as the seasons progress.

Mixed agriculture allows us to select the most appropriate ways of using land. Just because we're not very good at it isn't an argument for abandoning animal husbandry. Simplistic sloganising from vegans or vegetarians should always be challenged. They've made their choice, good for them, but there are perfectly valid and moral other choices. Eating less meat, eating local meat and always insisting on the highest welfare standards is a good start.