Our bodies are fickle machines. By maintaining good posture and square breathing you trick your body into the impression that you are in control, your heart rythm calms down, that in turn calms your mind. It's a bit like the perfect opposite to hyperventilation. Also meanwhile you can concentrate on something else instead of working up anxiety. It's not practical or relaxing, presumably the best breathing is allways the classic deep breaths "through" the stomach, but if you are overflown with anxiety, that makes you "take it all in", though by that point a true nervous nelly is most likely not taking it all in and rather feeling "knotted-up" somewhere between lungs and throat. I know I know it's all mildly esoteric but the asians were onto something.
I guess you're right that you can't do it should anxiety suddenly hit you in a stupor, without "loosing face" but then again it's better to stammer and breathe wierdly than pass out, back in school we had a few students who were notorious for fainting in front of crowd.
edit: Why not "take it all in"? Well you can and presumably in the long term, if you allways choose to, it's best for you knowing yourself. The question is do you have the time to process your emotions before giving a presentation, if you do cudos, you'll presumably be a better person for going through life living every moment fully conscious of yourself, without slipping through the hard times just hoping they'll fly you by. But sometimes one just wants to perform, and there is allways time for soulsearching later.