Rabbits and rodents don't have molars for grinding. It's only the front incissors that have a very effective overlapping cutting action that keep growing. No need to keep grazing if humans had them. We'd either be visiting the dentist as regularly as we visit the barber or we'd have a Dremel tool in the bathroom beside our tooth brush. I think not to be overlooked here is that it's the anantomy of our mouth, teeth and tongue that enables us to make a complex range of sounds that enabled us to develop speech.