I think mouth breathing definitely has utility when our bodies need elevated levels of oxygen (such as during rigorous physical activity). However, breathing out of line with your metabolic needs seems to have a detrimental effect (i.e. mouth breathing at rest). Apart from the increased oxygen, the nose also filters and humidifies air as it passes to the lungs. I'm by no means an expert on this topic. So if you know anything to the contrary, please do share.