From a naive min-max approaching to building your diet, it generally pops up that -some- meat is super useful. Simply by doing that you ease all sorts of constraints on your diet, and allows you to optimize your veggies/grains for other stuff, instead of desperately trying to get all your iron and B12 in. Even from an overall energy budget point of view, -some- meat is super useful. There are large amounts of marginal land that is not really useful for large scale cultivation, but perfectly usable as feedland for free ranging animals.
