not only is it a health concern AND an efficiency concern, but it's a motivational concern.
when you tighten up and
lean forward, you reduce the intensity of the exercise - similar to hugging the console on the stairclimber: you're reducing the weight load. you are also restricting the movement of your ribcage, so you have to pant and gasp to get enough air, rather than breathing smoothly in through your nose and out through your mouth. finally, it'll cause tension, pain, and eventually soft tissue irritation through your neck and shoulders and who wants to work out if it's going to hurt?